Have you tried using a gamepad? I don't have MS, but I have my own issues that make playing with a keyboard and mouse extremely difficult- my hands tremble too, my fingers and arms will cramp extremely easily and I have to stop every 15-20 mins to massage them and keep a heatbag nearby. Im prone to making many mistakes with keyboard/mouse these days that I never used to when I was younger but Im able to play just fine with gamepad. Im regularly topping dps, MT in events and solo healed plenty of things in 2.0-3.0. There was no way I would have been able to do that if I played with a keyboard.

If you havent tried it, I know a lot of mmo's have really awful gamepad setups, but XIV's is great and easy to use. Like I said I don't have MS so maybe my experiences would be a lot different to your's, but its worth a try.