Update 3
OK. So I spent at least about 10 hours so far the last days now and I´m starting to lose my motivation really now.
Screenshots show offset for second use case (using headset head tracking). I did 5 step calibration and everything is ok. But after restart of SteamVR there is offset again – even with taped proximity sensor. It seems like I have to EXACTLY be in the same place I did calibration everytime I start SteamVR. So annoying… 🙁