All Calendar events & calendars GONE

For a few days, starting a couple of weeks ago, iCloud was asking me if I wanted to sign in... or telling me I couldn't sign in, or something like that. I kept closing the window since I never use iCloud, and hoping it would go away. Around a week ago I signed in for a minute through System Preferences to see if that would make it go away... it seemed to want to sync with things on my computer, which I DO NOT want, so I unchecked things like Calendar, Photos, etc. and signed back out. I didn't really pay much attention to it since I never use the thing, just tried to get it out of my hair.

That message, whatever it was about not being able to sign in to iCloud, went away, so I assumed that solved things.

However, I believe it may have caused the problem I am having right now.

Today I went to add an event to my Calendar, which I've been using for well over a decade—note, I probably haven't used it for a couple of weeks—and discovered that my custom calendars are gone and all of my events, going back all the way to forever, are gone.

There were a couple of blank default calendars named Home and Work "On My Mac" and that was it.

I looked for a solution on the web and saw a recommendation to delete the contents of the Calendar folder and the iCal prefs, and go back to the last backup in Time Machine, so did that—after deleting, I plugged in my external hard drive and reverted the iCal prefs and the contents of the Calendar folder to my last backup (which was about a month ago, and also before the weird requests to sign in to iCloud... figuring I'll lose a couple of events but not too many).

After I reverted it, as I watched, extra items appeared right back in that Calendar folder. When I went back to open Calendar, the problem was still (or again) there. I reverted again, this time with wi-fi off, and rebooted as well, thinking maybe the wi-fi was interfering. Again, the items I had *not* placed in the Calendar folder appeared out of nowhere, and when I restarted Calendar, it was the same.

I turned the wi-fi on, tried signing in to iCloud and making sure there were no weird settings, signed out again, and rebooted the computer. Still the same—only that brought folders named U.S. Holidays and Birthdays into the calendar (and removed the two blank folders which had been in "On My Mac").

SO

not only are my decade-plus of calendars along with all the things I've done in the last month "gone,"

BUT ALSO

reverting doesn't bring them back. And weird voodoo keeps making folders appear in the Calendar folder (and I think the iCal prefs are doing weird things on their own as well).

What am I missing? If I have a backup, why can't I get back to my proper Calendar folders and events?

———

2011 MBP using 10.11.6 El Capitan