Just so I’m not second guessing myself, if there was a destroyer blocking you, you could still perform an amphibious assault if you aided the transport with fighters, right?
you can always make an amphibious assault with a transport… as long as the attacker wins whatever naval combat occurs in the SZ. if a Destroyer and a transport attack a SZ for amp assault and there is an enemy destroyer there, combat occurs. as long as the enemy is sunk, you can continue to amp assault the territory. If its survives then your transports are sunk as they do not have any combat (or defense) roll.