Q: Magic Mouse keeps forgetting the tracking speed

Sorry, bit late to reply, Tom, but have you tried re-setting the SMC.

Reset the System Management Controller (SMC) on your Mac - Apple Support

If some older custom mouse settings app were in use in a newer OS X, I wonder

if that could create an issue. Here's an older freeware http://magicprefs.com/ that

says it works with Mt Lion OS X 10.8.5 & newer. (I'd tried an older OS X version.)

Not knowing if such a software were used to custom set a mouse preferences, or

if only those settings found native in a recent OS X were problematic, I'm curious.

Presently, my mouses are tethered by USB shackles. They're not going far.

I don't use magicprefs so I'm not aware of its facilities.    Funny you should mention it though as I was thinking of looking into magicprefs only this afternoon.   Have you tried contacting them?

Only looked into their site to see they still offer a product, for free...

However that item has no useful purpose with my USB mouses.