[Oso954]

Totally wrong screws. They should be 32 threads per inch, and are typically about 1/2-3/4 in length. You have probably bunged up the receptacle threads so badly that even the correct screws will not work.
Do not waste your time trying to take them out. The only thing I’d do is cut the head off if I wanted to save the wall plate.

Replace the receptacles, use the correct screws.

 

[Oso954]

That receptacle 15 amp duplex white, not tamper resistant is $0.71 at my local Lowe’s.
$0.61 each if you buy the 10 pack.
Getting 10 or more, plus wall plates, would not be a bad idea. I’d wager he has some more old ugly receptacles around the place.