1. I haven't got the exact formulas for both at hand, but don't think so.The city SAM has 50% non modded games (IIRC regardless of defense of the plane), for flak it is 9% per flak against the unmodded standard bomber (up to the counting maximum of 4 AA units per tile, which would have a chance 36%), for a single mobile SAM I guess it 18%.Perhaps best would be finding the thread..."AA formula revealed" or like that.
2. Defintly no, they have their chances for every incoming plane.Figthers are limited to one interception mission per turn, however.
3. Don't think so, just balanced the way it is.Would be rather unbalancing IMO, because the AI hardly builds and uses them.