You can specify the W100 as the temperature sensor associated with the E1 thermostat. It doesn’t matter if you configure the buttons yourself.
However, as soon as you connect the W100 to a thermostat or group of thermostats, you will need to use the buttons to adjust them. Therefore, it makes sense that the buttons cannot be assigned twice.
Ideally, I would like to see a switch on the W100 that would allow you to change the mode directly between thermostats and individual-button operation. Unfortunately, that is not possible at the moment.
On the other hand, you can also configure some of the buttons for lighting and some for temperature settings. For example, button 1 could be used to turn the lights on/off and button 2 to set the temperature to 22 degrees. I don’t think it’s possible to increase or decrease the temperature incrementally. This would only be possible if you use Home Assistant or something similar. In the Aqara Home app you can only set specific values or turn on/off the thermostats or turn on/off smart schedule.
By the way, I also control my lighting with the W100. I just made a video about it. Maybe you’ll find it interesting: