try to peel up the bottom and fix it without taking off the skin. I'd try to peel it back just on the bottom around where you'll be fixing it and have a look to see if it will be possible to fix it that way, if not then plan on the bigger repair. Seems like a
2x4 clamped to the top surface would help while peeling the flange up so the skin doesn't distort. You can also just cut away the flange and fabricate a piece that looks like the flange and reweld it back on after the door frame is fixed. I dunno... just shooting around idears.