1. Can a plane in a coastal area such as england defend in the seas connected to it?
No. Only the zone it is in. The only way for planes to defend in naval battles is if they are fighters on a carrier/
2. can submarines submerge after their combat phase
Yes.
3. Do planes attacking on a coastal area like an island such as midway conduct both land and sea assaults
No. The Planes can either be part of a land attack or a naval attack, not both.
4. Does a ship in a coastal area defend the land or island next to it on an amphibious assault.
No. But remember that if you attempting an amphibious assault, you must first clear the sea zone you are landing from from any enemy ships. So in this way, they can prevent an amphibious assault. For example, suppose you are attacking Germany with an amphibious assault and he has a destroyer in the Baltic Sea. You must first sink his destroyer and then you can assault Germany with any transports that remain. But if he manages to destroy the invading fleet with his destroyer the assault is defeated. On the other hand if you are assaulting a island like Britain with multiple bordering sea zones, you can conduct your assault without worrying about a fleet that is on the other side of the island in a different sea zone from the one you are assaulting from.