Time to upgrade your wallet to something that has RFID protection. Seniors are prime targets, we normally have more disposal income than youngsters, are less likey to notice credit card theft, and generally not as savvy about tech.
I don't think there would be enough tension on the retainer to pull the screw out. I also am not really in favor of using the Sanberg retainer and leaving the G without a retainer; your most recent photos show the G string closely matching the break angle of the E and D strings, I wouldn't be...
Or you could drill a new hole in the retainer at the common slot. You could also cut off the tabs on either end, giving you a one hole three string retainer.
How about a custom piece that attaches to the present button retainer, either from above the button or under neath.
That headstock makes me wonder why they bothered with the G but ignored the A; strange happenings in River City.
All I can offer you is the advice that has been repeated over and over; 1) loosen the strings, you don't need them fighting you. 2) Bend the neck slightly back to take ALL the pressure off of the nut. 3) Use lubricate (you've done that). 4) If your wrench still isn't loosening the nut, more...
Get a wide rubber band (elastic) and stick it between the screw driver and the screw. If that doesn't work drill the screw head with a small bit(the size of the screw shaft). In either case, replace with higher quality screws or even better, cap screws. Stop torquing down on the screws so...