As a general rule I keep all my saving stuff turned off, but my auto save set at: Save every 60 minutes. That way if I get so absorbed into the game it saves for me, and I think, "crap, I forgot to save, thanks for reminding me!!"
I usually save every 30-45 minutes, with two saves to have back ups. Then when I'm done with playing the game I go to the save file and delete the oldest one. The more saves I have (this is I assume the same with everyone else) the game lags more. The next time I play I make an extra save and play on, saving on top of the new save (and the old save depending on what's going on) to back things up. Then repeat.
With that system I haven't had problems. As for the problem you have... I can't remember seeing a pit above the exit. Then again I don't play the main quest much. The others seem to have solved or attempted to solve the problem. Telekinesis seems to solve a bunch of problems. Telekinesis is... I believe an Adept level spell and if you're trying to get it from Tolfdir I think you need to be in the 40's in Alteration? Or maybe it's in the 30's... I can't remember.. Otherwise, the places they have listed are locations you'll find the tome in.
Hopefully we helped you, sorry you got that problem though. :/ I've had my own experiences of accidentally pressing a button and something bad or wonky happening. Best of luck to you though.