@gerry69 Conclusion:

I thank you all for trying to help me!

As I was about to reinstall again (!) I discovered Recalbox as an alternative distro and thought I'd give it a try (forgive me). While on Retropie Vice refused to install, and lr-Vice had to be compiled 3 hours for stuttering results and Sega Mega Drive was unplayable with both emulators, in Recalbox EVERYTHING works perfectly smooth without even the need of setting up or installing anything.

So I hope you all understand that at this point - after at least 20 hours of trying to get Retropie going - I give up and go happily with Recalbox.

Thanks again anyway - cheers!