Not being able to bridge into an amphibious assault after clearing a sea zone results in a pretty weird rules glitch. It results in people doing some strange moves like Italy building a destroyer and dropping it onto an oversized US fleet just to take advantage of this glitch. It makes sense that transports cannot unload / amphibious assault if the sea zone has enemy surface ships present, but not being able to bridge into an amphibious assault once the sea zone is clear doesn’t seem to fit the spirit of the game.
What you’re advocating for is the ability to have a second Combat Movement Phase. That would be no different than taking a territory in combat and then combat moving units through that territory to attack another territory. You have to make all of your combat moves at the same time before you begin combat or that breaks the spirit of the game.
I’m not sure how many different ways he has to explain to you that loading and unloading a transport into a hostile territory is a combat move. Your assertion that a ground unit can step on one end of a transport and step off the other end into a different territory is perplexing. It’s not a bridge as you contend, it’s a ship that transports units across the water. It makes no difference whether the destination is in the same sea zone or 3 sea zones away. Bridging is simply a term that’s used to describe the move. We also use the term “can opener” yet no cans need to be opened to accomplish it. It’s just a term.
In your example you state that dropping a Destroyer in water to prevent a fleet from amphibiously assaulting in incorrect. The attacking player can kill the Destroyer and assault the territory on the same turn. The thing they can’t do is load the transport in that sea zone. The rules of the game are very specific and at times a little quirky, but they are fair for both sides. You have to learn to use those quirks to your advantage in order to truly appreciate them. It’s called strategy.
Here are some of the quirky transport rules demonstrated;