I've had some pieces of fabric that wouldn't true up no matter what I did. I think it is from difficulties/tension in the manufacturing process and I can see how it would be even more problematic with the extra width.
I am a believer in prewashing, for many reasons but often it helps get the fibers back in line.
In your case, I would simply do what you can to make the back as smooth as possible and don't worry too much about the straight of grain. For a lot of us older quilters, straight of grain used to be a very big issue. I think though with machine stitching and such, it is more of a hold-over from our hand sewing techniques. After all, we can fussy cut or do all sorts of things like hexes where "straight of grain" is only a concept.