Try view>threads>expand all

It works! Thanks so much. A quick follow-up if I may: is there a way of applying that setting to all folders simultaneously?

Yes, sorta'. It requires careful keystroking: To set many *existing* folders, follow these steps: This must be done on an account by account basis.

1. select table view - the icon just to right of 'quick filter' button above message list pane
2. select a message folder and apply all desired attributes - View>sort by from menu bar (if not shown, click alt key)
3. click the icon at right side of column display in message list pane
4. select the entry near bottom 'Apply current view to...'
5. highlight 'folder and children'
6. move mouse left to see account name and move left again to display all account folders. Now CLICK. a small menu will appear to confirm setting all folders within that account.

Thanks: that went beautifully until the very final stage: the thread display was established for all folders but the other settings in my original folder, such as which column options (Starred, Attachments, etc) appear or do not, didn't get passed along. I'll experiment further...