I understand your frustration. I think your problem may be the direction that your seams are ironed. If you look at the back in photo no. 6, your seams are not nesting and that can cause your sewing machine to push the seam out of alignment. You could iron the seams open and try again or you could pin pin pin--one pin thru the exact spot the two seams should meet and another pin about 3/8 inch on EACH side--then sew very very slowly.
To get rid of some of the bulk in the center, swirl the seams. It really does help.