A-body armrest - stripped threads in door’s sheet metal

I guess I’m being slow. If I understand you correctly - I’m to cut a slot above (and not touching) the oversized hole. If I insert a large screwdriver in the new slot - am I to push straight down on the screwdriver to elongate the slot & force material down into the existing hole?
Exactly!

Very easy fix if you have access to a Mig welder.