Since you booked with a ta they have to do all contact for requests/changes with disney. Now even though the slot is full as she stated, she can still put a request on your reservation for a change. The paper that was sent to my ta(which she then sent to me) has this number 1.800.511.1333(reservations team)and it states to call this for quetions related to"your" clientss dining and show schedule. There is a 5,7,9 show times and 6 & 8:30 dinner.
So have her put a note and then try again when you get on board.
Sorry for all the extra stuff,might help someone.