The US cannot win this battle. The fighters will continue to defend in the air until the battle is over. The Japanese fighters and US subs can’t hit each other, so the fighters will sink transports until the US retreats. If the US wishes to conduct this amphibious assault, it must bring units that are capable of hitting the fighters.
This is one of those unique situations where you are required to take Trn as losses before subs because that is all the unit can hit?
Yes. They just need to be the only legal selection for a loss. That might mean that planes have no other target (like this example), or that Sub hits have no other ship targets, or that the defenders hit everything else in a round of combat and need to assign some more hits before the defender retreats. Remember, the defender still has the option to retreat, if they can, and transports are only automatically destroyed when they’re attacked, not when attacking (unless the attacker has no place to retreat).