The whole "apprentice" mechanic is mechanically interesting to me, even if there are disappointingly few decent builds that make use of it. (I believe I've done dwarf gadgeteer before.)

Some Mook Valkyrie variants that can be useful in some parties (particularly those that have a Gadgeteer to cover X-Ray, but lack Mage spellbook access):
* Change to Bishop at level 4, change back at level 7. This will allow the character to learn Missile Shield and Enchanted Blade, at the cost of some HP and 1.5 levels worth of offense. At caster level 3, the failure chance of PL7 Missile Shield or Enchanted Blade can be reduced to 12%, which is reasonable for s spell you're only using outside of battle, plus you can get a few extra SP from the extra known spells.and spellbook skills, Power Strike at level 20, Power Cast still 17 (IIRC).
* For a non-Power Cast build, raise STR/DEX, at least until DEX reaches 55 (longer if you want). Stay at level 6 or so in Arnika, but do get some spellbooks, including Enchanted Blade. Go to Trynton, use the Trynton fountain for +5 INT, use the Trynton fountain, then level up to Bishop level 3 (level 9 if you stayed at level 6 earlier). (Consider using spell picks on mage-only spells, as you may need them for practice.) At level 9, you can now learn Enchanted Blade and Missile Shield. (Optional: Learn Detect Secret.) Then, after learning those spells, change back to Valkyrie.
* Spending more levels as Bishop gives more spells. 5 Bishop levels can give you Knock-Knock, and 8 can, with a lot of extra practice (during which time you can't level further or you'll lose the ability to learn non-Priest spells), allow X-Ray. (X-Ray doesn't need a higher power level, and for locks, 18% fizzle for Knock-Knock isn't too unreasonable; might not want to use it on traps on Iron Man, however.)

Falling Stars is easier to cast at higher power levels than Earthquake, as the difficulty is computed by adding half the spell's SP cost to the spell's level. (I found the information elsewhere, and may put up a separate post about it.) (I have been studying it, and am not sure if a level 24 Priest with max skills can completely eliminate the fizzle chance for PL7 Falling Stars without a Geomancer's Ring; if what I read is correct, however, PL7 Death Wish's fizzle chance can't be eliminated by any means.)

Power Cast is useful for Superman (can give you 40+ for all stats) and Heal All (can restore 80+ HP to the entire party at once); it also helps Heal Wounds (less important because Restoration exists, but Restoration comes after Power Cast even for hybrid builds that get Power Cast later (but start raising INT right away)), and Guardian Angel. (Apparently, it's something like a 26% bonus to spell magnitude at 100 skill.)

Thing is, ignoring spellcasting, Bishop is better than Mage; in particular, more HP and better equipment options (in particular shields), while the 5% resistance that Mages get is minor in comparison. The comparison to Psionic isn't quite a clear cut, as mental immunity can be useful in some areas. Alchemists and Priests get more HP than Bishops, but Bishops still have better equipment than Alchemists. (Priest and Bishop have mostly the same equipment.) Furthermore, in the Priest/Bishop comparison, the only things Priests have going for them are faster leveling, the primary skill bonus, more HP, and Pray for a Miracle; both classes get Dispel Undead, have the same equipment, and use the same poor attack growth. (If Priests used the same attack growth as Rogue/Bard/Gadgeteer, there'd be more reason to use Priest over Bishop, but sadly this is not the case.)

Yes, I think Bishops are worth the penalty, as it makes them much more flexible than specialist casters, especially if you're trying to use an element the target doesn't resist as much. Also, Bishops get more SP than other casters because of being able to learn more spells and benefit from multiple spellbook skills in the SP calculation.

I note that even with the slower leveling, a Bishop is still going to be better than a hybrid; when doing equal XP comparisons, and assuming sufficiently high levels, the Bishop is only 2 levels behind a specialist, while hybrids are 5 levels behind in casting abilities.

Toying with the idea of a Dwarf Bishop:
* Gets 6% Damage Absorption, which might help with survivability, along with 60 starting VIT. This could be increased slightly later on with VIT boosting gear. (Checked in game. DA seems to be VIT / 10.)
* 60 VIT means more HP, which should help with survivability.
* Extra fire resistance (minor, but still a little bonus).
* Gets Power Cast at 16, and Power Strike at 18. (Compare this to 12 and 17 for humans; not much difference for PS, but is a big difference for PC, and is the reason why this race isn't used for more conventional magic focused bishops.)
* Speed is low, but that can be addressed later, particularly since the only interesting pair of boots boosts Speed by 10 and is buyable.
* Spellbook skills start at only 5 before bonus points. This means that, to learn level 2 spells at level 3, 2 skill increases through use are needed. (Wondering if starting as an apprentice has something to do with that.)

So, we get here a battle Bishop that doesn't die as easily, but will take longer to become a truly impressive spellcaster. In particular, through the low and mid teens, this character will be lacking in terms of spell effectiveness (though getting Diamond Eyes early-ish might help, or you could just use the Staff of Doom).

Ninja or Monk levels for stealth works better than pure hit points ime but I always move any Bishop to the back with extended weapons as a matter of course so a frontline Bishop sounds messy ;(

It would work better for survival, but it comes at the cost of caster levels, and each class added to the mix adds more stat requirements to juggle, which makes it harder to get the desired expert skills.

In nay case, I really do want to try a frontline Bishop and see how well it works.

If you really want to train a frontline Bishop on close combat and or ranged combat skills, I noticed a good respawning safe place: In the Upper Monastery Library (where no magic can be cast), you can stand just outside of the doorway, put the Bishop in front, everybody else on defend. Cast Guardian Angel/Bless/whatever on your frontline Bishop(s), then whack away at the rats. They will respawn every 24 hours sometimes, sometimes more if you're lucky. The other 2 spawn points (the garden before Altheides and the room where you get the Lightning Rod) have fixed monsters that don't move.

I'm not too crazy on class changes because of the requirements to meet that new class. If I build a character, it's tailored for that one class, and usually won't meet attribute requirements for any other classes, at least at the beginning. Not to mention, any levels as a non-Bishop class will cost spellbook levels (it's hard enough for a Bishop to learn their spells on time for 4 spellbooks, with the possible exception of Alchemy and mixing potions for skill gains and profit). For that, Burz is your friend for the early game; Buy him out of Light/Mod Heal potions and whatever else you can mix, sell the better potions, rest and repeat in a somewhat safe place (is the room where you need the key to unlock Resurrection potions, with the Cleaver, a safe place?)

Ideally, I would want a Human Bishop (2 possibly, for 2 portals and the ability to cast Element and Soul Shield on round 1). My reasoning is as follows: I want Powercast (boosted by the Trynton Fountain of course), and Snakespeed early. I believe that this will happen at level 12 and level 17 respectively. Immediately after that, Senses will be maxed (same Initiative bonus as Speed), and Piety for SPs and Stamina, quicker Divinity and realm skill development, and Iron Will. These 4 should be maxed by level 29 or 30 IIRC, if Senses and Piety are kept about equal. By level 30, you can work on the other 3 (Vitality first priority for HPs Stamina and CC) and Dexterity and Strength are last priority (IMO fighter stats). Probably best to put 2 per level into Strength, Dexterity and Vitality at that point. As a Human, you would have the most total attribute points to spend on a class that needs so many attributes, minus the small benefit of racial resistances, that won't matter that much with +10 to every realm's resistances (100 in realm skills), Iron Will, Element and Soul Shields up.
I know that I'm probably going to get flak about putting Piety ahead of Vitality, but I consider it priority 5 for pretty much every class, because there are so many ways to give yourself "more HPs". You have: AC from the heaviest armor you can equip without going into the blue encumberance, Shields (maybe even Light Shields!), +3 AC from Amulet of Healing, +3 AC from Cloak of Many Colors, Missile Shield, Armorplate, Magic Screen, Soul/Element Shields, Bless, Guardian Angel, Superman, Haste (high speed gives better AC), Body of Stone, Reflextion from maxing Dexterity, and I probably forgot something. You probably get my point. Vitality is the only way to get HPs (besides leveling up), however, it is a controlling attribute for literally NO skills. Also, Iron Skin is an OK expert skill; It seems to rise slowly, and can only be increased if you actually get hit.
This may sound like a small gripe, but if you somehow maxed 5 attributes, you would only have 2 attributes to put points into at level up, with no freedom to min/max: You would have to put 3 points into both attributes. If you have 4 attributes maxed as a Human, you could play with the numbers: 0, 1, 2, or 3 attribute points into each one, trying to even them out, or place them where you want to so you can ignore one of them. A striking example would be the Human Gadgeteer: I believe, from previous playthroughs, that you can get Dexterity, Strength, Speed, and Senses maxed at level 30 (in that order). You would then have: Intelligence 60 (+5 from Trynton Fountain, 55 to start), Piety and Vitality 45. Obviously, the order of importance would be Vitality, Piety, Intelligence (How does PC help a Gadgeteer?) The most useless stat is 15 points ahead of 2 better stats. From level 31 onward, the logical thing to do would be to give 3 attribute points on level up to Vitality and Piety (better HPs, Stamina and CC), and ignore Intelligence, at least until everything is even. Then, one could make an argument for 2 points to each per level up.
I wonder if it's wise to increase both Staff and Wand skill (using the 2-handed, extended range Quarterstaff that you start with), as well as Mace and Flail skill (hope that you get a Bullwhip, or buy one from Kunar or Sadok). This would seem to dilute the skills on an already overtaxed class. Perhaps just using the Staff occasionally to let it rise by itself, and putting a few points here and there into Mace and Flail on level up, is the way to go. By the way, should you work on Artifacts? It is tempting to pump it, since it gets a 25% class bonus. Not to mention, your Bishop(s) will probably have the highest Intelligence in the party, and decent Senses as well, the controlling attributes for it.
I wonder if I should play as a puritan with my two Bishops, saving all spell picks except Mind Stab at the beginning, which has no spell book. The Holy Water spellbook (level 2) you can get for free from the Savant Orb guarding the Arnika Tower (it may take a few reloads). The Banish spellbook (level 6) can be found in the upper Swamp at the crash site (it's hidden), or bought at a high enough party level from Braffit (I think around level 20). Minus Banish, there are 12 level 6, and 12 level 7 spells that have no spellbook, and must be picked at level up. That makes 24 spell picks saved, I believe that can be accomplished at level 24 if your skill levels are high enough.
This would mean that my Fighter, Rogue, Bard, and Gadgeteer (all easy levelers) would have to carry the 2 Bishops for the early game. My Bishops would use Mind Stab and swing their Quarterstaffs and sling stones until Arnika, where they could finally learn some new spells from Braffit and Anna. Then right to Trynton for the +5 to Intelligence, a quick visit to Fuzzfas for his limited spellbooks, then Crock and Sadok in quick order, then I would have to double back to Kunar. Does anybody think that's too much work?

Elf learns both those expert skills one level sooner (11 Power Cast, 16 Snake Speed); it's the best you can do for Power Cast on a Bishop without a class change.

By the time you get to Piety with your build:
* You have more than enough SP. Furthermore, the effect of Piety on SP is rather small at higher levels.
* You don't really need Stamina unless you're overloaded (in which case maybe Strength or Vitality might be the better choice for the load), or you're using a heavier weapon and getting multiple attacks/swings (in which case you should probably be boosting Strength
.

You can only get one Holy Water spellbook that way.

It's possible to get a second one, but doing so required killing Lord Braffit and getting lucky. It might be possible to pickpocket it, but pickpocketing is not that good in the current patch.

It is not possible to get any more of them.

Terror and Blinding Flash are useful early on, particularly in Arnika Road (and possibly a bit after that), when you're still weak and battles can overwhelm you.

Once you're past that, on the other hand, Terror and Blinding Flash are less useful; same applies to Blizzard and Pandemonium, though Blizzard at least does heavy damage (but you could skip it and go straight to Tsunami) and Pandemonium reportedly works against high resistance.

(Don't forget that you can use Flash Powder to cast Blinding Flash; giving some to a sling user might be a good idea for the Arnika Road trek, in case you need it; this can save a spellbook while still giving you enough access to this particular spell.)

This is the reason I don't like the Omnigun, or the Prismic spells from the Psionic; The effects are random, and might result in blinding or fear, which is what you don't want with magic users.

I could, perhaps, consider a party like the following:

In the front:
* Dwarf Bard, STR/VIT. (Reasoning here is high Stamina, plus the Iron Skin ability to stack with Caliban's Cuirass, and add Dwarven Damage Absorption on top of that. Might be fine with a different race, however.)
* Mook Valkyrie, STR/INT: goes for Polearms, but can also use the Giant Sword later on. Strongest melee attacker. (This version of the party has nobody else who can use this particular weapon.) While the Bard might usually get Bloodlust, it might eventually go to this character to increase Sword skill faster, if needed. Drawback is that Power Strike comes late, at level 19 IIRC. (Power Cast comes sooner, at level 17 IIRC.)
* Dwarf Bishop, STR/INT: Can be a decent fighter with these stats, while eventually getting Power Cast. More durable than the Human alternative, but Power Cast comes much later.

In the back:
* Mook(?) Gadgeteer, STR/SEN?: Capable of fighting at range, but also has the STR to function in melee and carry more stuff. Might frequently be using bows rather than modern weapons.
* Elven Bishop, INT/SPD: My standard Bishop. Power Cast at 11, Snake Speed at 16.
* Fairy Mage->Bishop (at level 2), INT/SPD: With suitable stat choices, it's possible to get both expert skills at level 10. Less powerful than the previous one in the long run, but I get two expert skills before the slog that is level 10 to 11. (Power Cast can be learned at 7, but that pushes Snake Speed back to 13. Snake speed can be learned at 9, but that pushes Power Cast back to 12 I believe.)

How does this look?

(Note that this version has the Fairy instead of the Ranger, making it quite close to a Magic Damage Party.)

Mook(?) Gadgeteer would stand up front with Bard fine ime

Are you going to pick spells for your Bishops at level up? Or are you saving them? Three Bishops means a lot of spellbooks to buy, the tradeoff being a ton of spells learned later on. This is almost like a MDP, if the Bishops and Valkyrie develop their magic properly,
Also, you have a significant range in experience tracks. Two specialists, one hybrid, and three elites. You will probably need to hold off on level ups, especially at the beginning, to allow the Bishops time to catch up.
You will probably go with Swords, at least for the Bard and Gadgeteer. Are you going to use Bloodlust for the tougher battles? The Bard can get a decent sword, Mercucio's Blade, from Higardi rogues.