chords are just a group of notes in succession, right? Well, I keep my entire thumb over the fret buttons, (the other fingers holding the back) and I can move it quite fast, partially from all the competitive smashing I've done. for seperated double notes I bring my pointer finger back around.
It is an unfixable problem. I DO have a right arm, but I have a mild case of hemiplegia, which is scarring on the brain that messes with the nervous system on one side of the body. so basically it has less coordination, slightly less feeling, less muscle, etc.
When I play GH, all my right hand does is hold the guitar up by the head. Do note the guitar is flattish over my lap. For smash, my right hand only presses A and B, so all my wavedashing, jump cancelling, etc. in melee I did with the control stick. I hit the c-stick with my left thumb too.