Thumb Keyboard solved!

If you've ever been frustrated that you get the "go-to-answer" on these forums about this issue -- simply turn your Duo and the Thumb layout auto-magically becomes available -- yet you already know it does not (and perhaps even mentioned this in your post), here is ...

The *REAL* Answer re: Thumb Keyboard Layout

Go to your MS SwiftKey app, the Layout & Keys section. Make sure the Extended Layout slider is OFF. Then you'll hopefully find the "easy" answer applies -- turn it and this layout isn't grayed-out. If your experience is like mine, having Extended switched on (which may even be its default?) keeps you from ever being able to select that layout, no matter what direction your Duo is turned.

I ended up figuring this out on my own because MS apps for work did some major installing on my Duo when I started a new contract/project, and (annoyingly) I had to go in and re-select all of my prefs for the work version of all MS apps/system tools on my Duo. In that process, I discovered that at one point I suddenly had the Thumb layout for the first time ever, and then on another day it was gone -- so I did some detailed trial-and-error with all keyboard-related settings until I confirmed that Extended Layout was the culprit.