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. If your experience is like mine, having it 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 app 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 all MS apps/system tools on my Duo. In that process, I discovered that at one point I suddenly had the Thumb layout, and then it was gone -- so I did some detailed trial-and-error of settings-sliders until I confirmed that Extended Layout was the culprit.