> 2) Do druids get anything besides the wizard and sorcerer spells? Most of my party is already one or the other, and I'm wondering if it's worth it to make them druids, or if I should just swap their spell type.

I'd just swap the spell type. I can't say that I ever found druids to be much use ...

> 3) I'm thinking of having mostly assassins with spell-caster abilities in the endgame, but would mean at least two retraining sessions. Is it worth it, or is it better to specialize fighter or spell-caster?

You really only need three (maybe four) assassins, as there's not much point having assassins at the back. Six characters with a full set of sorcerer spells are well worth having, though. How many characters you need with wizard spells probably depends on how much healing you want later in the game.