(1) "I must start work on our 2019 federal income taxes"
I'm also hard at work on my own taxes. Hopefully, they will put us into the same jail cell to pick up on Marilyn's emails afterward. I'll wait for you there, anyhow. And I wonder whether Greg is already there waiting for us.
(2) "Her email account...is in Mail "
I presume you mean in your Mail app (you signed in to Windows) & not just in her Mail app (she signed in to her user account & opened Mail). I want to know how you got her into yours, etc., as I asked earlier. Also, does it still sync, for example,
if she logs in to her account & deletes an email - does it disappear in your Mail too?
I must admit, I'm confused what happens, if you should (in your Mail) move something she deleted (in her Mail) back to the Inbox (your copy of her Inbox). Does it re-sync & get deleted again?
(3) " In WLM I could see what was in an incoming email message without actually opening it"
(a) I jumped from Win 98 (Outlook Express) to Win 8.1>Win 10 (Mail). But what you describe in WLM sounds like Outlook Express. There is nothing like that in Mail that I see. We must trust Windows Defender to keep us safe. There is an Email Security setting
but it says...

I only have the one Microsoft Account attached to Mail.
(b) If you think you can recognize a bad guy by its ID, you may click the "selection mode" toggle above the message IDs. Then you may select & delete them without ever opening them. Remember to toggle it back afterward.

(c) You can make rules in Outlook (on the WEB) that will be respected by Mail. But you'll have to know how to recognize the bad guys.
(d) We do have "View message source" in Outlook (on the WEB). Sadly, it looks like we must open the email before we can use it, though, so far as I can discover. Click the email in the top pane, then in the bottom pane above the opened email
click the More (...) icon to see it...

To get it outside the More icon (as I have now done), put a checkmark into "View message source", at "Outlook, Settings (gear), View all outlook settings, Customize actions". That will add the icon I circled above...

(4) "choice of closing all tabs or only the current tab"
That's up to the app. Right, Edge does that...

But IE11 (type into Search) does what you want...

In either case, you can just go for the little X's in the tabs instead...

Or use Ctrl+W.
(5) "Why were you working on the computer at three o'clock in the morning?"
That's how long it takes me to do things these days, especially in tax season & after a 3 hour nap.