Sorry you are having so many problems with your quilt top. Although I have no exact answer to your question, I find that although I try to sew an exact 1/4" seam, I can waiver a bit. I just finished piecing 42 log cabin blocks for our family's reunion quilt and after each round of sewing, I measured and trimmed where necessary, even if I need to trim only a small sliver of the block. I've been quilting a long time, however, I don't always get a perfect 1/4" seam, but by measuring each block as it is constructed I save a lot of frustration down the road. I may get a little OCD about this, but I seem to get better results. This is just works for me,others may have their own "tricks" to get accurate blocks. Hang in there.