Are the strategic attacks (Carrier Raids/Bombing/Sub) resolved immediately?
Well, they're resolved during the combat phase, at the same time as other attacks (see the third sentence of 4.9, and the note in the sequence of play under 2 A).
When a player performs one strategic attack may the attacked player's unit(s) defend and roll dice?
Yes, I think it's resolved as a normal round of combat. (4.94 mentions being targeted by enemy air units.)
Can an infantry unit using an amphibious invasion strategic move retreat back up to two sea hexes, or is it stranded and can't retreat after landed through sea at all?
It can retreat; see the Japanese infantry in the combat example on p. 7.