i have a friend that has done a lot of these. she has a long arm also, and does her quilting on that.she does stabilize every block-t shirt. it's a lot of work, and usually heavy. she's the type that pumps out quilts nd has done many and knows all the tricks to doing it.
from being in your position on more than one occasion of being remorseful. tell her you figured it all out, and it will cost more than she is willing to pay. if she asks, well, how much is that. tell her about $400-500, depending on the size. if she doesn't back down, get it paid half up front. tell her to pick her favorite 12, and you'll try to use those. good luck, and if you do it, show us what you've done.