Hiya-
1.Windows 10.
2. The navigation bar. might. have. been. Icons only. Not sure at this point.
3. This happened right after installing the update with Tasks, so I guess yes. There was the checklist feature box bottom left. A little confusing as to whether that was a tutorial or feature.
4. Restarting the app does not help. In addition, reopening the app selects the "Shared With Me" window, which is new behavior.
I have not tried restarting the computer as I am in the middle of work.
Hope this helps! Andrew