As you know you only need one nomination with a LOA. The SA will not assign / charge admitted students until very late in the process. They will continue to shuffle around who gets charged through May. You may never know where you appointment gets charged. You do not get the choice. Your only option now is to contact one of the the MOC, let them know your situation, offer to decline your nomination and allow the MOC to give the nomination to someone else.
Last year my son received a nomination to USNA and USAFA. He only wanted Navy. His plan was USNA, NROTC or NROTC College Program. The military branch, for him, was more important than attending a SA. When he was called and told about his nomination to USAFA, he thanked them for the recognition and politely declined the USAFA nomination, allowing the MOC to assign it.