1. A couple of times the active player moved into short range firing of the reactive player (to get the extra +3 FP). This often meant, the reactive player would take an opportunity action to fire (assuming they had an ‘unactivated’ unit and to also get the +3 FP advantage). We decided to resolve fire simultaneously, but the question is whether the reactive gets a shot before the active player, potentially ‘hitting’ them and damaging the active unit before its attack?
Defensive fire, if caused by a reaction to an action (like moving to the adjacent hex), comes first. The moral of this story is don't close on an opponent who can fire back... Approach from the flank (turning to face you is an action, and therefore he can't turn and fire), or on units that have completed their actions (they could use CAPS or Cards though), etc.
2. When taking an opportunity action to fire, can you use any remaining CAP points to increase your FP?
3. Combat: It says you can fire at a unit an unlimited number of times (as long as you have the AP’s or CAP’s to do so) but can you switch targets? Eg. Destroy a unit on your first fire, then turn, using an AP and then fire with any remaining AP’s at another unit within range?
Yup. Of course, the pivot, if needed, is an action in itself, and can be reacted to.