The Germans have no destroyer present, therefore the defending air units can not hit the sub, and of course the sub can never hit planes. Think of it in terms of the combined arms rule, air units can only hit subs if they are combined with at least 1 participating destroyer, the difference with this particular combined arms is that it works while attacking and defending. In the scenario that you described, the sub would destroy the remaining transport without rolling and the defending fighters can do nothing.
Defending AAA question
In the rule book it states that AAA fire back during bombing raids and nothing else, but does this mean that if you attack a AAA with an aircraft it is defenseless to it?
The AAA unit doesn’t fire during bombing raids. Facilities are assumed to have built in AAA which fires once during a bombing raid. AAA units get one shot at the beginning of a defensive battle involving attacking planes, and after that they can be taken as casualties as normal. If you attack an AAA unit on its own using aircraft, the AAA gets 1 roll to try and kill the plane(s). If it misses, then the AAA is dead.
Thank you that makes much more sense