Update: I could not accept defeat so I reread many posts on how to resurrect so-called "bricked" devices on this and and other forums. Re-educated, juvenated and animated, I was able to see the error in my ways, install the correct ADB drivers, connect with ADB, reboot into ap fastboot, use RSD lite to reflash my unresponsive XT926 back to 442 JB, then rooted again with towelroot. Whew!
I still want the native tether but now I am gunshy!
I'll try iBolski's modified tether process once I get an image of the rooted device...
Thanks to all contributors to this awesome forum,