Hope you are back on track! Thread basting is preferable to pin basting. (You probably figured that out now.)
Been there, done that, regarding the short backing. I once tried to make a quilt as large as possible, and had the backing too close to the length of the top--ended up adding a strip across the bottom. The backing will actually be a bit longer than the quilt top once it's all done, because it's on the outside of the sandwich as it rolls up. It's like rolling up a magazine and the edges of the outer pages never line up with the rest.