Respec for my priest.

[Xanthix]

Can anyone confirm Trolls have a racial spell called hex of weakness? Is it seperate from the warlock's Curse of Weakness? Can it stack?

Trolls do have this skill. I do not know the specifics of its operation, but the forum concensus (take it with a grain of salt) is that trolls have the worst priest racial spells.

Additionally, does spirit tap work if your team does the killing, but you only wand-tap the beast? I highly doubt priests would get finishing blow consistently.

You have to make the killing blow to activate Spirit Tap. If you practice with Mind Blast and wands you can get it a decent amount of the time, especially if your party knows what you're trying to do.

Regarding Devouring Plague, it does seem like a really nice spell, but I have heard that it is rather mana intensive.