This is why I'm waiting. I've had bad exp from beta software in the past. Everything works now so why take a chance and upgrade?

Nope

Can't one just take a nandroid backup of MM from the recovery before installing N and then restore the backup if needed?

(I haven't updated Mi box nor used the mi box recovery yet, so I am just saying how it works in tablets and phones)

Try factory reset. If you are not on the beta program then hopefully it will delete your update.

If that works and you don't ever want to update (for whatever reason), then maybe possible to block updates (via domain) on router.