i have had a similar problem and stripped it right down to the bare circuit board as the sun roof decided it would vomit lashings of water all over the mmi unit .. my whole system is down it wont get past the load up screen when it tries to boot up
did fix the button issue though before and after images
you can see the nasty corrosion on the system that makes the mmi think your pressing 3 buttons at once some times makes it shut down willy nilly and also boot up incorrectly
you can see the crud build up from water ingress or drink spillage
some of this with a soft artists brush did the job
any way it was simple
in my case sadly it did not solve my mmi booting up issue however i am slowly trailing through every optical component in the hope i find whats causing the issue now that my drains are clean at last
hope this helps ... might be too late but could be a sticky for future maybe