How do I force Single-Line layout to actually use a single line?

Solution 1:

In addition, you need to click the Message Preview button on the View ribbon tab and select Off.

Solution 2:

I started a new position with a company which uses Outlook 365, and DESPISE all of the bells and whistles Micro$oft dumped into Outlook. Geez, just give me back Outlook 97 and let me be.

Anyway, I think I found your answer here: https://www.msoutlook.info/question/message-list-1-or-2-lines

"...This is the Message Preview feature and this will prevent Outlook from showing in a true table style layout often preferred by people that have the Reading Pane on the bottom or turned off. To turn off the Message Preview feature:

Outlook 2013 and Outlook 2016 tab View-> button: Message Preview

Radio button: No AutoPreview"