That territory is considered full-friendly after you captured it during your turn.
So now that its after your turn, its defiantly friendly to the point where you can land planes there.
tl:dr Yes
I think you meant to say “definitely”
If a sub attacks a lone transport outside an enemy island with an airbase and fighters, is the transport automatically sunk, or does scrambling do anything to prevent that?
The fighters can scramble. However, since there is no defending destroyer, they can’t hit the sub. As a result, the transport will be sunk anyway.