Right Bumper only registers when pushed backwards and down

This sounds like it warrants a bit closer inspection. You said you opened the device? Can you post some images? The bumpers are actually triggered by a button mounted to the motherboard. Although those switches are reinforced by a bracket, it's still possible it is not firmly attached to the board any longer and pressing it at certain angles still allows it to be activated, or for switch to contact the board properly.

It's a fairly common fault on shoulder buttons in general, and I have seen it commonly on Steam Decks which use asimilar mechanism (albeit no additional brace).

Video below for some context. It should be timestamped, but skip ahead to about 12:20 for shoulder button info.

I would also make sure the little nub on the bumper that activates the button is in good shape. it could be bent, or deformed. Additonally it looks like those buttons are plastic welded into the front case, so those welds could have come loose as well meaning that the button isn't in proper alignment.,