Hi Volker,
I apologize for being persistent, but what I'm proposing is that you continue to provide the actual measured temperature to Apple Home (just like you do today), but in the Eve app you display the measured temperature adjusted with the offset in the at Glance and Rooms overview (today the actual temperature is not displayed at all in these Eve views, neither with nor without the offset). If you prefer you can make this configurable. Default is no display of actual temperature at all (just like today). Configured without offset is just like the Apple Home app, and configured with offset being proprietary to the Eve app. In my opinion this would be a great addition to the excellent Eve app.
Kind regards!
May 5, 2020