My guess would be that you can just add a longer screw to the butt pad adjustment screw and then install it there.
Or for that matter you can do the same to any of the screws available on the gun that are of a decent diameter.
I was heavily leaning towards replacing one of the screws that was on the gun! I didn't want to pick the butt stock screw because of how low it sits on the gun, and it may cause it to flop over, but that might still work since the front sling point sits higher. Hmmm.
I did some 2 point sling experimenting on my M3 and you are correct in your analysis. The best mounting points are going to be along the centerline of the gun (high), but using a high/low combo may work. For the impact front mount I am using an M-lok QD installed on the side of a Saber tactical ARCA rail. This puts the front mount at the bottom of the bottle (low). When I combined that low front mount, with a QD socket installed via the buttpad lock screw in the rear (low), the gun was pretty 'floppy'. I then threaded some paracord through the rear side plate slots in the M3 and mounted the rear of the sling this way (high-ish). That combo was ok. Considering you are using the Maverick front/side rail for your front mount (high), you may be able to get away with using the buttpad screw hole to mount a QD socket. Other than that, it looks like you would have to use one of those other two screws up top, or drill and tap the rear area some where. I don't see any convenient places to thread a paracord loop through which is the easiest cheat for sling mounting.
I bought the same QD socket shown in your picture and ended up using a Magpul QD socket instead for the buttpad mount. The reason is that the QD sockets are all designed for inch #10 screws and doing this mod involves using M5 screws. The Magpul socket is designed around a button head #10 screw, and an M5 button head screw drops right in. The other QD socket in your picture uses a special flat-head #10 screw that doesn't allow for an easy metric drop-in replacement.
The Magpul QD socket part number is MAG333 (Type 1)
The screw you'll need is a button-head M5x30mm or M5x35mm depending on what nut you want to use. You are going to install the new screw in reverse from factory direction, and then add a nut on the opposite side.
A couple thumb-nut options from McMaster (I like the first steel one best):
https://www.mcmaster.com/92815A202/ https://www.mcmaster.com/93886A240/ You could also just use a conventional nut if you don't adjust the buttpad, and that would be cheaper and easier, but not look as cool
