Can you become each class in this game (apart from 3 ultimate classes)? And have all spells and abilities?:)

What route should be chosen for this, considering you have limited choices when changing classes depending on your current class?

Zen master has access to all spells, so that's definitely a possibility.

I'm sure it's possible to train in all the skills as well, if you were to change roles enough.

But certain traits are intrinsic to certain races, so it would be impossible for a single character to acquire all of the possible traits.

Also, as far as I know you can change roles even after ascending to one of the elite roles. Would you want to? Probably not.

You can become:

One of the 4 starter classes (you cannot class change to a starter class)
All of the elite classes, depends on how much time you want to spend.
All of the special/ultimate classes (if you are female)

with one character if you really want to.

Many of the elite classes come with redundant abilities/spell-books, so you don't really need to do them all (but you can, if you are patient enough). Personally i would consider getting all the spell-books you want early and then mid-game switch to assassin which is pretty powerful and can be achieved relatively early, and late in the game change to zen-master (or maybe even valkyrie and then zen-master).

One thing to keep in mind is that if you change from a class with a certain spell school to a class without that certain spell school, the skill for that certain spell school is halved for the new class when considering which level of spells the new class could learn in that certain spell school when leveling up. For example if you change from wizard to paladin, having a sun-magic skill of 6, you will only be able to learn 3'rd level spells from sun-magic as a paladin (who does not have that school) since your skill will be considered a 3 instead of 6 when choosing new spells. I think the warlock is the only exception here - he always gets the full level when choosing spells.

Oh that's very important thing to note.

Also, the question: can the whole party become zenmasters (or valkyries, assassins)?

If you were rogue but became zenmaster, will you still have ability to lock pick staff as if you were rogue without any penalties, I understand it right?

Is spell penalty applied only to learning? Does mana pool increase for all magic schools if you will keep gaining levels as non-spellcaster?

The ultimate classes can only be acquired once per playthrough (you can restart the whole game any time and keep your party), since you need special items for them that you can acquire only once and they will be consumed in the process.

Penalties: you understand it right, as far as i know. Zen-Masters are as good as any rogue when it comes to lockpicking.

The spell penalty is (afaik) only applied to learning since the impact of spells is determined by the sorcery skill. Also you gain spellpoints in old magic-schools too when you level up in new classes.

It's possible, but not in a single game. Each of these special roles requires you to perform a unique task that can only be done once per game.

If you wanted a party of assassins/valkyries/zen masters, you would have to start a new adventure for each party member you wanted to ascend.

For instance, if you wanted 2 assassins, 2 valkyries, and 2 zen masters, you would have to play through twice. If you wanted 6 assassins, you'd need to start 6 unique adventures.

So if you start with 6 Wizards, then you can ascend to Samurai-> Ranger -> Monk -> Warlock, and thus have all the spells? And then you can change Warlock to any other class (like Ninja for "Lock & Traps" and then to Paladin to become like total superman) except those you already ascended too?

How important is "Lock & Traps" skill? It doesn't seem that aforementioned party will get access to it any time soon. Is it possible to just bash the locks and eat the damage from traps?

Edit: Oh, do you also keep the class traits when you change classes?

Edit 2: Also, if I start with all Wizards, will I still be able to learn Blacksmith trait in Warrior guild as advanced class?

In general - yes you can ascend through those classes to get access to all schools (samurai is not needed in your list). With a party of 6 you will need a lot of time for that.

You can get lock & traps from the thieves guild as soon as you reach guild level 2. You can bash the locks, but that triggers the trap, you can use the vinecraft spell "pry" to open the chests from a distance and dodge the traps, or you can use the vinecraft spell "disarm traps" to open the chests. All methods can fail leaving the chest closed and also trigger the trap.

You will keep the traits when you change class. As a general rule - changing classes never removes any skill or trait you already have.

You can learn blacksmith from the fighters guild (your wizards can just initiate the quest for samurai, join the fighter's guild and cancel the samurai class change afterwards). I'm not sure what guild level is needed for blacksmith, but i think it's 1 or 2.

One important moment for those who want to learn all skills.

It seems that if you multi-class too much and get all skills, you will encounter skill scrolling bug, which means you have too many skills to see them all, so you won't be able to level up some of them.

Skill screen shows a maximum of 20 skills, while there are 32 skills in the game. So if you want to max out all skills, this is the list of 12 skills which should be maxed before they become unavailable because of bug:

Traps&Locks, Pickpocket, Stealth, Forge, Artifacts, Enchants, Blessings, Gallantry, Prowess, Deathstrike, Incantation, Music.