1. Agreed, Revan was undoubtedly a better fallen jedi than Desann.
2. The describtion of the Valley of the jedi, is that the one who wields it's power is going to have the power unseen EVER BEFORE in the galaxy, to erradicate stars in a whisper, that sort of thing.
Now, Desann didn't have that capability, at least he wasn't skilled enough to use it. But we saw him pushing Luke, who could resist being pushed by someone who had, what was it, 200 planets of force users strenght (I don't remember this quote exactly but....) combined into one, and yet we saw Desann doing it like nothing. Also, keeping up with Luke Skywalker, who could move so fast he was barley seen and strike so that his sabers looked like more than one, is pretty good.
3. Actually, if you look closley, the two pillars that are on the far side of the room are already crushed, but the one close to you, is not. I belive they put those in to show, that Kyle really wasn't a match enough to kill Desann with this amount of traning, I mean, first invinsibility, then pillars to crush him with. If they just wanted a hard battle, shouldn't they just have given us a hard battle? Cause to win this battle is almost impossible if you play on challenging levels..... The fact that the game doesn't show his body when he's dead should prove this more...