1. Mostly just the ones from the game, plus a few that are not used in the game (but that don't contain any animations either). There are no orcs.
2. I'm not sure how easy that would be, since the animation pipeline has changed quite a bit.
3. You can adjust the size of any model very easily, there's a field on root templates that allows you to change the scale.
4. You won't have to recreate it, but I don't know whether you can just reuse the level. If not, an easy work around should be to export a level as a level template in a shared mod, then use this shared mod in both an adventure and a GM mod and instantiate the level template in both. You can customise the template (by adding things, not removing things) in both mods then.