@chris2vic, @Woo
Thanks again Chris and Fred for your help in debugging and your continuing patience.
We did not quite reach the 10 iterations before converging, so we can consider this transatlantic debugging, you with the hardware and me with the code, as a success ! 
To summarize what I learned thru this exercise : the Handler errors with the latest v2 Hub and latest Fibaro FGK-101, which appeared several months ago, came mostly from the new CRC16 encoding of Z-Wave messages.
Also, the Z-Wave Open/Close commands asynchronously issued by the FGK-101 when opening/closing changed unexpectedly from “Basic Set” to “Sensor Binary Report”.
Both did not exist 1 year ago with v1 Hub and previous Fibaro FGK-101.
Contrary to what I initially thought, the SmartThings v2 Hub is likely not the sole culprit for those upward incompatibilities, and a surreptitious firmware update for the latest versions of shipped Fibaro FGK-101 has also to be suspected.
Unfortunately, if SmartThings and/or Fibaro created such undocumented upward incompatibilities once, they can do it again… 
And since “SmartThings support doesn’t provide official support for creating custom Device Handlers and SmartApps”, there is not much help to expect from SmartThing support…
We reach there the limits of a supposed “open platform”, which was however one of the advocated strong points of SmartThings…
Regarding the last remaining Groovy error, you can ignore it since it is a part of the Handler code which is non functional, added to track some SmartThings random bug which occured in the past (the Wakeup period for the FGK-101 would randomly revert to some value different from the 1 hour explicitly set by the Handler).
I will try to notify SmartThings about this bug which is clearly part of their CRC16 routines, but considering the above, I am not even sure where to report my findings…
Regarding the °F setting, it was actually completely independent of those hardware/firmware issues, I just had never taken the time to code and debug it properly, as I was interested only in °C. Interesting thou that the °F<->°C dynamic switch is allowed from SmartThings mobile desktop, while SmartThings code for valueTiles does not support it.
But I suppose unless you use SmartThings from a boat or a plane, there are not that many cases functionally requiring °F<->°C dynamic switching while running the same Handler… 
Let me know if you find some other abnormal behavior… hopefully not too soon !