Good question. The only one I know for sure is that Dragon Priest are about a 1000 levels above you.

Stupid dragon priests.
All Dragon Priests are level 50.
I think in a lot of cases, but not all, the level range of the mobs is +/- range within 2 or 3 levels of the player character level when you enter a dungeon.
The game draws enemies closest to your level in a pattern - most of them are generic fodder with occassional "miniboss" and of course a "boss" at the end. But the enemies don't scale forever - for example highest non-Chief Bandits(Highwaymen) are level 25. So even when you're level 80 and you encounter a bandit dungeon you'll be still facing mostly Highwaymen simply because there's no "better" enemy for the game to spawn in the Bandit category.
So you could start to clear a dungeon, but leave and come back 10 levels later, but their level will be much lower, around what you were when you first entered?
Yes - the "level" of a dungeon is set in stone first time you enter it.