Well it might be that something changed in the X server and/or kernel that makes your combination unstable. I think you should definitely try the boot options and wait a few days for the new LTS kernel 4.8 - don't throw the towel in yet 
Maybe someone other from the forum will be more experienced with such setup or know what can be the cause, but from what you have written, I really think that the multi-device setup is the cause of your problems. Try it a few days with just 1 keyboard and 1 mouse and see how it works, or try to pull out and then plug in back the keyboards and mice once it freezes again. Again, it would be REALLY helpful if you could drop here the system log after the freeze (the similar excercise as you did when you had problems before, with syslog etc.)
Maybe someone other from the forum will be more experienced with such setup or know what can be the cause, but from what you have written, I really think that the multi-device setup is the cause of your problems. Try it a few days with just 1 keyboard and 1 mouse and see how it works, or try to pull out and then plug in back the keyboards and mice once it freezes again. Again, it would be REALLY helpful if you could drop here the system log after the freeze (the similar excercise as you did when you had problems before, with syslog etc.)