[camelreef]

Good. I think we have to end this here. There is nothing left I can do for you.

 OK... Would you care to explain?

And who can I turn to, now?    

 Your issue has zero to do with kodi. Whatever happens is only happening for you on your installed / configured system. Clean installations with e.g. LibreELEC proved that. We asked many others running Ubuntu (including me) to test your samples. No one had that issue (Edit and most importantly there is absolutely no correlation with anything in kodi's log.  

I can't fail you there... But one last question.

So I've reinstalled from scratch, bare server, then your guide, step by step. The only thing I've added is the xorg.conf file for the intel driver (though it looked like it was working with the modesetting driver while I had forgotten about the intel driver).

The only difference from the previous install is that I've learned about netplan and didn't force an ifupdown network setup. We learn everyday...

It looked like it was working very sweetly and crisply!

Then I started my Roon server install. There is a dependency on ffmpeg/libav-tools + cifs-utils. Could those packages be a source of conflict?