@bblitz:
1. Can a nation land air units on a territory that was captured on the previous turn by a friendly nation (…)
Yes.
@bblitz:
2. (…) can the controlling French player attack the sub (…)
No. Nations from same side band to defend, but not to attack. The French fighter alone can’t detect German’s sub. On the other hand, if the German Sub attacks during next German turn, then french fighter can scramble (giving the airbase is active) and defend as the UK’s destroyer will detect. French and UK band to defend together.
@bblitz:
3. can the attacking player decide to take the first hits on his carrier instead of the fighters knowing that the fighters will not be able to land following the combat due to the status of damaged or destroyed carrier?
Yes. See it as 3 different steps. For each plane, you have to plan a legal landing during combat move, you are allowed to presume all combats will be flawless for the attacker. Then combats are resolved, not binded to any landing declaration, you can take hits (even sink) carriers. During non combat phase, any planes without legal landing will crash. This is usally done to hold on better dice for remaing rounds of battles.