What would you do?

I think I would get a panel from another car or rear door like you said, and splice it into place. You'd be hours and hours ahead and would need a lot less filler in the end. Cutting that section out and trying to straighten it would be huge time suck. I love fixing things myself, but I usually try to find the easiest way that will give a good result and in this case, that section would be going into the scrap bin.