2018 GLC 220d electrical gremlins

Hi

over the past 3 weeks i have experienced these minor issues :

1. sat nav turned itself off , came back on 10 mins later
2. next day all radio stations had no reception. next day all working fine.
3. a week later the radio was on and all sound was lost. turned engine off and on then radio was working.
4. this week the car would not start from cold. I removed the silver start/stop button from ignition and inserted key into ignition and car started normally.

i put an alternator onto the battery terminal points in the engine bay (not directly onto battery) and it was reading 15-16volts. is this too high ?

any ideas as to what is causing these issues? main dealer said to photo/video these issues then book into workshop.

thanks in advance