What if the UK fleet unit is a carrier AND the US moves 2 Fighters in to the sea zone?
Do they have to move in during Combat Movement and fight the German fleet or can they move in during NonCombat Movement and just land on the UK carrier?
Well since I don't think it is in dispute that they can attack the German fleet we can safely consider the sea zone hostile. You may only enter hostile sea zones during combat movement (with the exception of newly built ACs but I don' t remember how exactly that conversation worked out.) So they can't go there during non-combat.
If they have to Combat Move and fight can they retreat after one round and land on the UK carrier?
I'm going to say no on this. As according to the rules on retreats you must abandon the sea zone contested.
There was a conversation about newly built ACs put into enemy occupied sea zones and moving your fighters to that sea zone to land on the carrier. I'm not sure what was decided on that or if anything came down from Larry Harris or Avalon Hill. I think the consensus was that this was the exception for entering an enemy occupied space during non-combat.
A friendly AC in the zone may be the exception allowing retreat to the space you are retreating from.
I would flip a coin each time this came up and use that for the rest of the game.