1) I'm pretty sure we haven't. Without code knowledge to know what is and isn't possible and what those factors are + the ability to search for NPC's I'm not sure how to could be sure. That said, new NPC's come in as well.

I never found a useful kobold, goblin, bugbear, jaette, or troll, but one might exist out there now.

2) Yes: You can only enslave hobgoblins, goblins, kobolds, gnolls, bugbears, orcs, ogres, jaette, trolls, and giants.
That's the message you get when you try to slave something that you can't.

3) No. Yes. Usually this ended up with a dead NPC for me. They aren't overly high HP (mostly) due to the rules for HP on orc-ish NPCs being weird...

There is something weird with aggro mobs and slaving. I'm not sure exactly the rules but some are impossible. I definitely have tried the trolls in Nonveil and failed. Dunno if level or something else preventing that.

4) Yes. Ogre Skjald's will be just as useless sing they don't actually sing. Which leaves mostly warrior specs for choices.