Update: All Sonoff S31 Zigbee outlets up and operating smoothly - Thanks for the Troubleshooting advice!
My observations: The Rebuild Zigbee Network should be used before doing the Pairing as it makes a difference.
As well, if Device is found but pairing fails, put another Zigbee device, even if not yet paired, next to it in pairing mode, as they seem to help eachother out... It was the case, twice, in my setting up of 8 S31s.
Thanks again!
