Thanks a lot for your response.
I managed to get things running pretty well by now and wrote down what I did, in case someone wants to do the same: Integrating the Homematic smoke detector (HmIP-SWSD) into openHAB - Tutorial & Channel documentation
There are a couple of remaining aspects which I do not yet fully understand about the channels of the smoke detector (see bottom of my post, especially around “COMMUNICATION_TEST_REPEATED” and “RESERVED_ALARM_OFF”). So if anyone has an idea, let me know. I’ll then update my post accordingly.