This is kind of why I stress staying with your current class. My MDP will have low HP, but their magic abilities will be awesome once I reach level 24 and can cast level 7 spells at PL7(I believe, because their 25% bonus to spellbooks gives them a huge bonus that they otherwise wouldn't get being another class).

Then again, with Mage or Psionic, you could get more HP without losing casting ability in the long-run by starting as a Bishop, then changing to a specialist later.

You probably mean STARTING as a Bishop, and then changing to a Mage or Psionic, for the extra HPs. Bishops have a heavy experience penalty, but that doesn't matter so much at early levels.
Doing it the opposite way, for example making my level 19 Mage and Psionic Bishops now, would probably be bad, as I will really feel the Bishop's experience penalty at this high level. Not to mention, NO skills in any other spellbook. I would be casting the other three spellbooks as if I were a level 1 character(the spells not in the mastered spellbook.) Plus, I would lose the 25% bonus. No thanks, I won't be switching this late in the game.

No, I mean starting as a Mage, which will allow you to start with more INT than if you started as a Bishop.

Also, the idea is to change at level 2, so the character only loses one level of non-Mage spellcasting, in exchange for getting Power Cast early.

Starting as a bishop, then changing, is a reasonable option, but has the drawback that Power Cast still comes as late as it does on a pure Bishop; it's probably better suited to a late game class change, however.

Fighter->Bishop->Fighter, or alternatively Hybrid->Bishop->Hybrid, is an interesting option for parties that would otherwise be lacking critical spells, but don't want to dedicate a slot for a pure caster.

You have been on my thread, and you know that I am putting off maxing Intelligence for Powercast, because I know that it will catch up very quickly, as PC skill increase triggers on any spell cast. Snakespeed, on the other hand, increases much slower, which is why that was my priority.

If you prefer Snake Speed, then the Faerie Mage->Bishop is still a good approach; you just allocate the points a bit differently. 77 SPD is possible at level 1, and 77 or 76 SPD will give you Snake Speed at level 9, at the cost of Power Cast coming later. Drawback is that Faeries can't equip Snakeskin Boots.

Alternatively, I believe a Felpurr Bishop (no class change) can get both Power Cast and Snake Speed at level 13; slower than Elf Bishop for PC, but much faster (3 levels I believe) for SS, so that's a viable option. Note that you don't get to wear the Fey Ring, but you do get to wear the Ring of Beasts (which is probably easier to get).

I actually used to make my Bard and Gadgeteer Felpurr for the same reason you suggested(fast Speed max and Snakespeed). I have since come to learn that Snakespeed is great on everybody, but only really important on primary spellcasters, especially Element/Soul Shield casters.
So far, the only enemies that I have faced that can compare to my Initiative, are those level 10 Higardi Brigand. They are not a threat to me by now(level 20/19 party), but they can still be annoying as to who goes first(best to use hit all spells here).

What I used to do, for optimum expert skills, was: Lizardman Fighter, Hobbit Rogue, Mook Ranger, Felpurr Bard, Felpurr Gadgeteer, Elf Bishop.
After many years, and many playthroughs, I learned that reaching expert skills early wasn't the only important thing. The focus, now, has been on longevity, which is why I migrated towards Humans, with their highest stat totals.

Mutli-classing in this game, with a few exceptions, seems to be an inferior strategy, as each class/profession is tailored to a certain skill set.
For example, if I wanted my level 20 Mage to switch to a Bishop, provided I met the attribute requirements, all of her other 3 spellbooks would be functioning as a level 1 character, only able to cast level 1 spells at PL1.
Even staying within the framework of the Wizardry spellbook, if I changed her to a Samurai, her weapon skills would be starting at 0(Mages can only work on Dagger, Staff and Wand, and Throwing and Sling skills), without a shield even. Her HP would be terrible for a Samurai, who should be fighting on the front line.
Bards and Gadgeteers function as magic users in respect to their Music and Engineering skills. For example, an instrument or gadget that mimics a level 7 spell will have a level requirement of level 18, and a Music/Engineering skill requirement of 90, just like a caster. This only means, at level 18, that they can safely use it at low level. I'm not sure of the exact game mechanics, but to safely use high level instruments and gadgets, you would think that you would have to be at level 24, and have 100 Music or Engineering skill, or you risk fizzles and backfires(especially bad with Asphyxiation gadget!)
Someone told me a while ago that, in order to safely cast level 7 spells at PL7, you would need to be at level 24, and somehow have your magic spellbook, or Music/Engineering, above 100 skill(easy for a specialist caster to pull off with their 25% bonus to spellbook), not so easy for a Bard or Gadgeteer. This would lead me to believe that a Bard or Gadgeteer, to safely use high-end gadgets, would need to push level 30.
I seem to recall a long time ago that my Gadgeteer completed her Asphyxiation gadget in the Sea Caves, and despite being in the level 20s range(tough enough to defeat Nessie!), she kept backfiring at my party, killing a few. This seems to strengthen my argument that it isn't a good idea to change a Bard or Gadgeteer to a Fighter at level 18, you would do best to "stay the course" and keep them in their profession.

You can get 110 Music skill on a Bard with the help of a helmet that Higardi rogue types sometimes drop.

Also, I don't think there's any benefit past level 24 for the success rate of spells/instruments/gadgets, though such a high level will still help with piercing enemy resistance.

With all this said, the risk of a NegatAir backfire isn't that bad at level 24, and you can still mitigate it:
* Save before the fight, and only use the NegatAir early in the fight, which is when it's most likely to make a difference. If you get a nasty backfire, reload.
* Alternatively, put up Magic Screen beforehand, and then Element Shield in the first round. Then, if the NegatAir backfires, it won't be that bad, and it's likely to not kill anyone; if somebody does get killed, just revive them (you *do* have that Resurrection spell, and some Resurrection Powders just in case, right?).

I believe that you are talking about the Puck's Cap. I think it's cursed, and although it does grant Music +10, it has an attribute penalty of Piety -10, lowering your effective Stamina. I don't think the AC on it is that great, either. Bards can get the Bascinet from Antone early, which is their heaviest helmet for a while.
It could be that leveling past 24 could help make up for not having your spellbook/music/engineering skill above 100, though I'm not sure. I will have to test this in Arnika with my pure casters. Right now, at level 20, they can safely cast level 6 and below spells at max power level, and their hit-all level 7 spells at PL3. The Bard and Gadgeteer have their Music/Engineering at skill level 95, it's becoming harder for them to level that higher without cheating(casting the Charm instrument repeatedly at He'Li, for example). Hopefully I can get Music and Engineering skills up to 100 by level 24(they're 21 now).
I know that having a high level, by itself, is a huge advantage, nevermind any attribute or skill levels. I wonder if a multi class Bard/Gadgeteer turned Fighter at level 24 would count that higher character level(level 30 for example) into piercing resistances, or it would stop at level 24.
Yes, to your point, I just gave my Priest and Alchemist the Resurrection spell at level up. I'm picking all the level 6 and 7 spells now that you can't buy, waiting on Braffit to sell me Banish. My effective party level is 20 1/3, probably rounded down to 20, so I'm at Braffit's mercy on that one.

The Bascinet doesn't provide anything other than head AC, and AC is boring and not even that important later on.

My Bard and Gadgeteer have an AC of 19 right now, that's without any buffs cast. I suppose that is decent for second-rate fighters(they're both considered Burglars, and don't fight as well as Fighters or hybrids, but better than pure casters). The Puck's Cap has an AC of +3, has Earth resistance +10, and is non-Fairie, as with a lot of good items. This is according to Flamestryke. This is compared to the Bascinet, which has +6 AC, and doesn't drain your Stamina by taking away Piety.
I guess we can agree to disagree. That's the beauty of this game, there are so many different ways to develop a party, giving everyone a way to play the game as they see fit.

The Earth resistance is probably better than the AC, as it protects against spells like Crush and Quicksand, which is more of a worry than physical attacks that happen to target the head.

The Stamina loss from low Piety is minor, and it's not as bad as a STR or VIT loss would be.

There's also the weight issue. I've found that, later on, I prefer to use lighter armor even on the classes that can wear heavier armor because the heavy stuff just weighs down my party too much. (At least it's not as bad as Final Fantasy 2, where heavy armor ends up, from a defensive stand point, worse than no armor at all in some fights (including the final battle), and also hurts turn order and agility growth.)

AC is really not that good; I'd rather have the special effects than just get more AC.