SUCCESS!
When I pulled that bolt, it came out easily. But the spacer slid down and covered the threaded hole. Because there was no natural friction to hold the spacer in place, I had to struggle for about 30 minutes to use a long thin screwdriver to manipulate the spacer, reaching in from the left side of the bike. Then I used what fingers I could to help align the bracket and bolt and spacer. Finally, the bolt slid in. I tightened it a little and the went to the other side to start that bolt -- only to discover I had the bracket on upside down. (Of course, this IS the 3rd Monday of the week!) First, the spacer was the problem, then it was the space between my ears that hindered me. Well, 10 more minutes and it's back like it belongs. Next time I have that bracket off (which I hope is never), I think I will wrap the spacer in foil to help provide a slight bit of thickness so the spacer will stay put for the next reinstall. Knock wood!
Special thanks to Dan, ST11Nut. He called it.
Thanks to all of you who offered advice. So grateful.