Don't know if this will help but I am on my 3rd Magic Mouse and only because I got one with the new 27" 5k iMac but I had 2 earlier ones I loved, on my MBP, both different OS, and after several months, each mouse just stopped working.
I was able to take it off the one it was set to and run it on another but no matter what i was unable to set them back to the original machine I had them set to.
I've had my 5k iMac since Xmas and I started to notice yesterday one key would not do anything when I hit it. Somehow I got it working and am noticing tonight that I am able to press the 9 key but I cant capitalize the same key for the bracket above. I will give it a day or two more and if it continues I will take it back to Apple for I have 3 others here on my various MBPs and never had an issue with those keyboards.
I remember when I had issues with the magic mouse before, there were a few having problems although I still think its a small percentage......