I thought I might flip the bottom/back over to eliminate the mirroring, and create a vertical strip to use at the edges. I think if I create the edge as a separate image, I can split it in half vertically and paste and blend it in. My thinking is, that it would be easier (for me) to blend the edges of the strip in towards the center, and the outside edge will automatically match up, because of the way I split it. Hope that makes sense. Something like this:
_A B_
(ER EL)
_C D_
E right and left represents the strip, split, overlaid and blended into the four main parts. I think your suggestion to add a little blur at the edges would help, too. It would leave a little "fudge factor" for the graphics engine, and soften the seams.
Wish me luck.