truly even a commander can rescue troops (did it myself) .. but that is because commanders retinue are troops that can be shooted at too, yours probably vanquished as the enemy unit shot them down.
Have not seem yet artillery being tactically captured (that is, usable by your side in the middle of a battle instead of guns being available at camp) but this is indeed ok, even if defender do not disable the guns , horses would be missed, and few infantrymen would be able to man the guns properly.