Switched to coreelec 9.0.1 simply by putting the coreelec update into the Update folder an then did a reboot.
All Problems gone,
CEC Poweron works
no flashing white bars on the right side of the screen anymore
no sudden stops on live tv or recordings
no skip problems anymore
I also switched to CoreElec 9.0.1 and have working CEC. Everything else seems to work too. Excellent!
I was not able to upgrade by just putting the update in the .update folder. I rebooted and the update process seemed to proceed. There was an error, however, and my odroid failed to boot, so I had to do a full installation and that worked. I restored the backup I created of the .kodi directory and my library was intact. I don't know if I'm going to run into problems in the future by doing it this way but we'll see.
Hopefully this gets fixed in libreelec but it seems I'm now a coreelec user...