There's a reason why hundreds of years of philosophizing has never satisfactorily answered those types of questions 
Philosophy has a ton of answers to those question. Just pick the one that satisfies you the most.
Can't. I have no idea what I'd do until I were actually placed in a situation like that, and I hope it never actually happens.
Ah, but if you decide what you should do in these situations, in the unlikely chance they'll actually come up, you're less likely to do something you'll regret. So it's not worthless to think about, despite the unlikelihood.
Well, I have to admit that what I'd do would probably depend on which lives I had to save/condemn. If, say, the one were my wife/lover, while the ten were child rapists, I'd probably let the ten die and not need as much therapy.
Please note that all of this is purely hypothetical