As if Dave ever gave a crap about the stability or lack thereof of loops in time. He threw his own time-shifted corpse out a window, after summarily deciding that he didn't want to die that way after seeing the body. Just up and there fucks over causal-linear time. He's Dave, he can do that. Hell, I'm surprised he isn't carrying around photos of things so he can keep track of how the future (and past maybe) changes, since he is an amateur photographer and all.
But that's just it; the dead dave was from an unstable time loop, and was thus doomed.
No, the entire event was unstable. You got to flip it cause-ways. The loop is supposed to work like this - Dave walks in, sees lack of books, goes back in time a few minutes, gets shanked by DD, a few minutes later the scene plays out again. Instead, upon seeing one of himself, Dave decides not to do the obvious and forgoes any time travel. That means the play of events that should have created the corpse he threw out the window never happened, because he talked himself out of it upon seeing a corpse that had no causal reason to exist.
Which I guess indicates a non-discretionary persistence of matter across the fourth dimension, and puts the kibosh on my
Back to the Future photos idea. Pity. The universe really does eat paradoxes for breakfast.