Thanks @ElectricSeal – appreciate the followup. When I originally followed your clarified instructions, still didn’t get it to work.
I decided that I would try reinstalling 1Password in the event the MAS update didn’t go quite right, but was prevented from deleting the app because it was “still running” even though I had quit it. I hopped into Activity Monitor and, lo and behold, it still had an open thread running (along with the Mini browser). I quit the former and force-quit the latter, then tried your steps again … and Bob’s your uncle.
TL;DR: Fixed, but just be careful about silent open threads when you think 1Password isn’t running!
Thanks again for the help!