Well, the explanation I heard is that snoring provided protection during sleeping to scare away predators. I don't know the source for this theory, so take it with a grain of salt :)
I heard that too, but it doesn't sound likely. If I was sleeping and there was a predator passing by, I would prefer it didn't notice me... :)
The fact that heavier (and thus more attractive as prey) people are more likely to snore could give credibility to your explanation.
Most likely, yes, like the loudness of baby crying. Humans are pack animals so any predator attracted by snoring or baby cry, and deciding to check it out would be in a very very big trouble.