I cleared Embershard mine and I've gained a lot of levels since. Lots of time gone by and it's still empty.

Do they respawn ever?

yes...maybe. is happened to me. a half dungeon respawn.

I think so. Dragons also seem to respawn that protect word walls.

Yes they do respawn, after a while though not within a week of time in Skyrim.

Yeah, I've had bandit camps respawn. Annoying when I spent ages slicing their shiney cockles in the first place!