I have two Fabaro door / window sensors installed on our front and back door. I am interested in using the potential free input to connect to a switch which is triggered from the manual door lock. Has anyone done this successfully and where do I find the binary flag which indicates if the switch input is on or off? Also can this input be used concurrently with the door sensors (magnetic) and temperature sensor DS18B20?

It can be done, I have no personal experience so I can’t answer all your questions, but there is a blog post (in swedish) that maybe of interest.

The input is in parallel with the internal sensor, so you can’t use the magnet if you are using the external switch. You should be able to use the temperature sensor and the input at the same time.