the tac bomber can only defend against the enemy ships conducting the assault, they cant attack any transports.
if the attacker brings a ship and doesnt clear the sea zone they can’t amphibious assault.
if they dont bring any attacking ships, the tac bomber can’t scramble, and the transports unload normally.
The Tactical bomber can scramble if there is an attack from an adjacent sea zone, this includes unescorted and defenceless transports landing (attacking) during an amphibious assault. You’re right that the landing can’t take place if the sea zone is not cleared, and the fact that transports alone can’t destroy the tactical bomber, the logical response would be to retreat ASAP.