I can likely answer whatever you need. If you can post a few pics I will be happy to walk you through removing the plague that is trying to attack your car.I have not personally, but @Revvv has experience in the body work field
I would order a new rear quarter panel from Mustangs Unlimited. I would personally cut out the areas marked and used them as a template to cut those sections out of the new quarter panel. Test fit, tack weld, then butt weld everything in to button it up.