Hmmm... things seem to be working.
I had been trying to do all this via an user I set up with Admin privileges, which I had been using for years with no problem.
I revived my poor Admin user, gave him Admin privileges (was a user), and can now edit other users. I still get the Internal Server Error when I try to edit settings, but if I edit a setting, hit the back button, and reload, the changes have been made.
So things don't seem so dire.
On a side note, I made sure I was updated, and among the files instructed to upload were these two, which I can't find anywhere, not on my downloaded version or the MLF website:
So I think things are sort of kind of OK. It would still be nice to be able to import a database into a new install, but I'll keep playing around with that on a test site.