OK, the best update that I could put in here: works like a charm! It just took ages to detect the switch which BTW I placed right on top of the hub (to quote a Youtube video that I watched earlier today "proximity is everything with Zigbee… LOL). Once I had it on top of the hub it took good 5 minutes to detect it but it did with the device handler I published before automatically.
The rest was very straightforward. Putting it in pairing mode is done pushing the button for 20 seconds and since the switch is battery powered you can bring from where it is and put it right on top of the Smartthings hub.
So to summarize if someone ends up in this thread:
- Create the device handler with the DTH and instructions in the Third Reality FAQ
- Make sure the switch is on pairing mode (very straightforward since the led is blinking and there’s only 1 led. If not in pairing mode, push the button for 20 seconds until it does blink. More details in the user manual.
- Place the switch on top of the hub or as close as you can
- Add as a thing in the app (more like search for things)
- Wait, Wait, Wait, Wait
- When detected, rename if you want and click ok.
- If the handler was not used (you will notice right away because of the default device join name for the switch in the code (RealitySwitch) go back to the developer portal and edit the device to use the handler.
- Voila
Thank you so much @Automated_House for making go back to do this with reassurance about the ZHA 1.2 protocol 