1. I guess it depends on what you want to do. Beaches and natural beauty is Maui or Kauai, IMO.
2. That doesn't seem like a big deal. There's plenty to do in Hawaii that doesn't involve strenuous physical activity. (I would avoid the big island for this reason, BTW). You're only there five days anyway.
3. For that short of a flight, first is a waste of money, but that's only my opinion. If I lived in the LA area, I'd probably fly Hawaiian out of LAX. I tend to like their service better than the legacies and I believe they serve Maui, Kauai and Honolulu all direct from LAX.