Thanks. I knew I'd seen an option referencing the Back button somewhere, but couldn't remember where it was.
As it turns out, I already have both those options set as you recommend. But the app still quits if I hit the Back button [even a short tap] from the accounts pane.
Tap Accounts from side menu:

Tapping back button from the Accounts window quits app, instead of [expected behaviour] returning to previous screen with side menu:

This happens even with the recommended options in Settings:

BTW. The above scenario is not an exhaustive list. I've just hit the back button from a screen where I had an alert asking did I want to save or discard a draft and that also quit the app. Again, it was only a short tap, not a long press. There's definitely some inconsistency sprinkled throughout the app, as to what the Back button is going to do, which makes it a bit of a guessing game.