I’ve got the answer to your prayers I believe. I have pictures to add to help show my explanation but posting them is difficult for me. I can email them.
I’ll try my best to explain. If you were to sit down and look in your wheel well lip near where the fender meets the side sill. The inner lip of the side sill is narrower in about a 1+” long area near the upper end of the sill. Take and remove the screws that attach inner fender shroud to the sill and a couple of them that actually attach to the fender itself.
Using a plastic “stick”, carefully pry back the inner fender in the area where the side sill lip is narrowed down. It only needs to be moved a little. Look behind it and VOILA! There is the nut that you have been fighting. The narrowed lip allows a ¼ drive 10mm socket to fit onto the nut.
Please note that the nut does not have to be removed all of the way. Back it off enough to allow the side sill to drop off of the “nub” that is on the bottom edge of the fender which fits into a slot in the sill and properly locates the side sill.
Hope this helps