What if the more powerful resurrection spell fails?

(Also, are you sure the character's vitality (or whatever the stat is called) doesn't decrease on resurrection? It does decrease in classic Wizardry.)

Now at level 3 i get two points :D, maybe there a is maximum limit depending from character level?

Do you know how the attribute points are instead calculated?

For example, i levelled up my dwarf warrior and i got 3 hit points and 1 attribute then i reloaded i got 7 hit points and 2 attribute points, re-reloaded again and got 15 hit points and 1 attribute point.

The attribute points seems to vary between 1-3.

They indeed vary between 1 and 3.
But it doesn't matter much, because you will be able to raise all of them to 18 and use ankhs to raise them even further, to 24.

One of my warrior is doing the samurai quest, but he could also take to the Barbarian quest

If i start to do the barbarian quest i think i'll finish it before the Samurai one, so the warrior will be like Barbarian -> Samurai.

But can i switch back to the barbian role after finishing the samurai quest?

Once you have been one of the elite classes (like Samurai or Barbarian) and change to something else, you can't switch back unless you reset adventures.

Wizardry 6 has a mechanic like this (and 7 might as well); however, in those games:
* This mechanic won't boost skills that are already above 50.
* The points aren't free; they are taken from the skill points that you normally get at level up.
* Which skils get increased automatically depends on the character's class; a pure caster, for example, automatically get their spellbook skill increased.

Perhaps the mechanic in Wizards and Warriors has similar rules (though with 50 obviously being replaced with a lower number)?

For learning Renewal quickly, is it fastest if the character stays a priest, or would making a monk speed things up?

How about if my goal is, instead, to learn every Spirit spell on that character?

1) This is not so trivial to answer (is it faster to stay priest or class change?):
To consider is that you have to be level 7 to get level 7 spells.

If you have lots of money, it's easy: level up to level 7 and pay for the training each time and you are done.
Otherwise (and this is the usual case) you will do a mix of training and level-ups without training because it is too expensive. If you change the class early, you need less xp for the level but get only 1 skillpoint, which if combined with payed training might get you quicker where you want to be, if you optimize it. (You do the math...)

But keep in mind that the Monk ascension is a pain in the arse: you need to spend 2 game days naked with your character and every time you check back whether you finally have fulfilled it, the time resets to zero (so save before you check!). Using DXWnd and its time stretching feature makes it a lot easier BTW.

2) "how can I learn all spells from one school (spiritcraft) the quickest?" Depends on what class changes you do. if you start with a Priest and change to a Warlock, you have no penalty for learning spells in spiritcraft, so this might be the optimal way since you level up quicker with the 2nd class. This all depends on how high you can get your spiritcraft skill, so again, you end up with the question "how can i get enough money for training?".