That's amazing. What A stroke of bad luck for you.
I've followed,participated in every thread on the Explorer/Excelleron I've found and don't remember anyone have this problem.

I have alwasy held the wing panels of a new ARF together side-by-side while aprecaiting the build quality. I didn't have a reason for it but realise now It's a good idea as I would discover a difference in length immediately by doing so. Sorry I know that doesn't help you in any way

Just a thought but would extending the wingtip of the short panel help. It would be much easier than cutting the wing in half.

I think if it was me, I would just relocate the existing wing cenetered on the fuse and fly it.

Good Luck
Eddie