Having one collapse and the other DOA sounds thin odds. Guess you'll need to test the replacement in the other PC as planned prior to any further actions. MoBo stuffed is another possibility, testing with an old(er) card might be inconclusive. Especially for a mechanical problem with the slot.
Nothing you already did not know by the sound of your post, wish you success.
The troubleshooting routine in a repair shop would be to take everything out of the case. Lay the parts on top of thick cardboard + foam, antistatic bracelet, and do an open-heart surgery. Not only is this a huge time-saver w.r.t. the multipe re-assembly efforts when swapping parts, but imperfect mounting to the case fooled quite a few before. Exchanging PSUs (or at least different rails/leads) becomes a quick task as well, which should the next step be on your list anyway.
EDIT: Given the cost for the GPU and your own description of what's been tried, IMHO you are well beyond the point of letting some local pro earn his bread.
Last edited by FlightDetent; 18th January 2021 at 08:33.