No, you are not required to attack with your naval units; they can just park there and wave politely. If you do attack, then yes, you may elect to retreat after any round of combat is completed (i.e. after the defenders roll).
I think you are asking about reinforcing a contested territory as the player who initially possessed it. Either side may reinforce without combat, as the status of “contested” gives no preference to the side who owned it previously.






