One solution for a quilt that need narrow additions on each side is to use scraps from the front and piece a matching border on the back. I think it would look cute.
As for the one seam on the back, I was not aware of the strain on the seam, so I have been doing mine with one seam for 30 years, and have never, repeat, never had one pull out, break a thread, wear wierd, or anything else. It works well for me. And I would rather hand quilt over one back seam than two or three. But, I also do whatever saves the most fabric, unless I want to use the backing for something else. That's my opinion and I am sticking to it.