Finally, I have figured this out.
I see that the information in the manual at http://127.0.0.1:36787/v12.30.1/#!Documents/Prefs/Folder_Formats.htm , which contributed to my confusion, has not been changed to indicate that Flat View overrides other formats. I hope that will be changed so that others might find that information.
The other issue I described above concerned a format 'sticking' after I used Flat View, even when turned Flat View off and navigated to another folder. I now know that this is why:
(from Folder Formats: Detailed Guide)
I now realize that turning on 'Flat View' and then selecting 'Images' from the Folder Formats menu has the same result as "making changes to the folder format", and that's why the change 'stuck' - even though I did not change a folder format, I just selected another."