Everyone:
I thought I'd start a new thread, just to let everyone know I had a solution to the problem of F24 starting with a command-line login, not the graphical login I'd been used to.
I'll pass along these two commands, which come from Garry T. Williams:
sudo systemctl --force enable sddm.service sudo systemctl start sddm.service
Run the second program only if you haven't started X already. If you have, do a restart.
I had found sddm.service disabled on my setup. Maybe it always was, and I never gave it a second's thought.
Anyway, F24 now starts as it should. And one thing I got back, is a full-sized "firetray" icon for either Firefox or Thunderbird. The latter is important: I now have back an accurate count of my unread messages, and can make Thunderbird disappear and reappear at will. Firefox, also--but Firefox gives me no other indicators.
Thus far, aside from a very old off-line password manager that no one has maintained since F12, everything works as I'm used to seeing it work. I'm running KDE, and now I get a complete KDE login, and can directly select a user account, type in a password, and sign in, exactly as I do with Windows. (WinDoze?)
I must say this dnf system upgrade program works better than I anticipated. And much better than fedup ever did. It shows me my progress in a way I can understand.
Thanks to the development team, and to all of you for your helpful suggestion.
Temlakos