Circumventing the "Last Name" problem
I think I've got the fix:
1. In Mail app, select drop-down menu "Window" (at top of screen), then chose "Previous Recipients" & go through and delete all the iterations of emails sent with the last names added.
2. Go to Contacts app, open each name where you don't want a last name showing up, and change each person to a Company (click Company under the name), then type in their first name as the name of the company. Oddly, you must also add their first name right under the company name box, too, but then it seems to work!
So far so good for me...