I am not yet ready to get this game (there's no temporary discount and I still have other games I want to play, including Tangledeep and a replay of Dragon Wars), but out of curiosity, I did look up what classes and spells exist, and there is one thing I am wondering:

* Is an Archmage build viable?

I define an Archmage as a character who knows every available spell in the game. In this case, it means having all six schools of magic on one character, and then actually learning all the spells. (How does spell learning work?) To make things more interesting, let's try to do this without using any of the Elite classes (I get the impression that they're more of a late game thing, plus you can only get one of each per playthrough, and other characters might need them).

In any case, it looks like the build might look like this:
Wizard (only way to get stone magic under these constrants) -> Paladin/Monk (I'm thinking Monk, as it looks like that class has the more useful trait, plus I want healing magic (which is unfortunately confined to one school; I would prefer it if that were not the case) -> Ranger (need vine magic) -> Warlock (gets remaining spells, and apparently can learn spells from all schools easily). Does this look good?

(By the way, how does spell learning work in this game, anyway?)

What you've suggested seems viable.

All spellcasting classes have, at minimum, 2 associative skills: suncraft/stonecraft/mooncraft/etc. and sorcery. The skill tied to the school of magic you're training in (e.g. suncraft) dictates the highest level of spells you can learn. Sorcery determines the magnitude of all your spells.

It won't work exactly as that.
Wizard will be able to change to either Bard, Samurai or Warlock (classes, which have affiliation with Wizard guild). But once you will choose one of those advanced classes, you will get access to their own basic guilds, so then more advanced classes will be available to you.

But your initial choice will be basically from Bard, Samurai and Warlock..

You can check this link to see guild affiliations.
http://www.jeffludwig.com/wizardswarriors/classes.php

It's a roundabout way, but he could accomplish what he laid out initially.

Take the quest for Warlock from the League of Sorcery, then join the Brotherhood of Promise.
Cancel the Warlock quest and ascend to Monk.
Take the quest for Paladin from the Brotherhood, then join the Clan of Three Swords.
Cancel the Paladin quest and ascend to Ranger.
From there, he could restart his quest for Warlock and complete his training.

It sounds a bit cheesy and feels like exploit to bypass class change limitations.

It is exploit in the same way as exploiting character creation to make gold. Though I must admit that starting as naked character with 200 gold (which isn't really enough for anything) is somewhat ridiculous.

So, if I were to make the character a Samurai, would I then be able to do Monk and Ranger in either order?

(Of course, that means picking up a trait I don't want on the character; is there a limit to the number of traits, or is it OK to have a wasted trait that does no harm?)

(Warlock would be last for this build, because I want to be able to continue learning high level spells.) Whether someting is considered an exploit is, the way I see it, subjective.

For example, the makers of the Bard's Tale Remaster took away the ability to start intra-party combat because they felt like using it for "endless healing" is an exploit; I happen to disagree (it takes too long to be useful past the beginning of the game anyway).

Dragon Wars has a New Game + feature that is available from the start (you keep skills and spells, but lose equipment and quest progress); is using it to get multiple copies of the Inferno spell an exploit? How about having multiple characters wrestle for Druid Magic? How about using the spot that gives everybody 5 skill points (equivalent to 2.5 level ups) repeatedly?

It souds like this game also has New Game +, and I read that you can't return to an old class in the same game. If you restart with the same characters, can you return to an old class then?

Yes, you will be able to do Monk and Ranger in either order.
There is no limit to traits, you can keep them all at same time.

Is there any way to get rid of unwanted traits? (For example, the Barbarian's trait looks like it has a drawback, and the benefit would not really benefit a mage character, for example.)

I wonder about it myself. This is also the reason why I will avoid Barbarian. But if I am not wrong, some traits aren't passive abilities, but active abilities.

So I also wonder, is "Fury" a passive or active ability?

Fury is a passive ability. It is always active. At least i can't activate it anywhere with my barbarian.

Thank you for making this clear.