I think they should still cost 5. The Wizards.com FAQ for revised (2004 edition) says AA guns can target fighters or bombers separately. So the AA guns are no stronger in AA50. They are actually weaker because they no longer fire on planes passing over-head like they did in revised.
Just as other ships may, submarines may also move out of a sea zone where they begin their turn with enemy ships, even destroyers. The destroyer doesn’t stop the sub’s movement, as the sub didn’t enter the sea zone - it started off there.