Messing with time is dangerous. And even if you went back in time and stopped some horrific event, there are no garauntees of a happy ending. Case in point, harry went back in time to save sirius from from the dementors kiss. BUT, it still did not save him from death. What you expect to change only has a short term effect. The long term is still very unpredictable, and i believe, no deterrent to fate. I think to theory of snape using a time turner is to complicated. It's got to be much simpler. Dumboldore is dead, Snape is on the good side. It is the game and the sacrifice....For both of them.