this was a break down from the beginning
i don't even know a LA that would use sheets for the backing and i certainly have never known a LA that would be willing to piece a back for the customer.
if you wanted her to piece your back it was up to you to tell her and then at least a conversation could have taken place on the issue.
then the LA should have called you and said your back isn't big enough for the top bring me a different backing fabric when you can and the top will sit here until you do.