The Overlord can spend experience points on the purchase of treachery, and utilize that treachery to acquire appropriate cards for his use in dungeons. In RTL, is the Overlord allowed to spend the treachery on corrupted (dark) glyphs, and use them in dungeons as well?

I think that you can. There is no restrictions in the rules.

Yes, you can. However you are limited to using it once per dungeon, not once per dungeon level. Personally, the glyphs are only so so from the few times I've used them. I would suggest using crushing blow and dark servant ( I think, its the one that switches a nonmaster on the board to a master) you will get more use out of them than a corrupted glyph.