Now I dont know if you want to try this or not it worked with a lap quilt I did last year. For some reason my tension got off on my Model 15 and I had something that looked really familiar to yours. I always check my quilts as I do them but it was 3 am and I was on full speed ahead mode so I could hurry up and get it done.
The next day I looked at the back and saw where it was all buggered up like yours and I really was not into taking it apart. That and the fact that I had to wrap it up and put it in the mail for a gift that morning..........
So I took the quilt and tossed it in the wash and washed it on hot and dried it on high and the threads shrunk and tightened up and I did not have to fix anything!! But I never pre wash anything before I make a quilt.
I dont know if you want to try this or not but it might work for you.
Billy