Your quilt is beautiful!!! I don't know the answer to your question, but I can tell you that I helped hang quilts at our local quilt show, and many of them didn't hang straight. I do't know why, but I thought at the time that was just the way it was...that gravity was at work or that our rods to hang them on were at fault or something. We hung quilts "back to back" and that seemed to help some.
Not much help. Sorry. If your quilt is square, it almost has to be the sleeve, but you have already thought of that...
Dina