Just as other ships may, submarines may also move out of a sea zone where they begin their turn with enemy ships, even destroyers. The destroyer doesn’t stop the sub’s movement, as the sub didn’t enter the sea zone - it started off there.
Even if those extra Fighters survive the battle and then have to crash into the sea?
Interesting stuff! 🙂
The thing is, you don’t know if those fighters will have to crash into the sea. They may end up on non-combat move without a valid landing spot but when you perform your combat moves there’s always a statistical change that the carrier will kill the 100 battleships (even though it should be about the same as you de-materializing while reading this message and materializing on the next room) thus the combat move satisfies the rule of a valid landing spot (regardless of how improbable it might be).
Combat moving 2 extra fighters to a battle goes against that same rule because there’s simply no possibility for those fighters to land during non-combat as they are moved (unless a carrier magically pops up out of nowhere into the middle of the battle board - odds for that happening should be the same as you teleporting yourself to another room) .