iPhoto Upgrade Catch 22
Turns out that the iPhoto version was being reported incorrectly. It was really version 9.0 (from the iLife '11 DVD) not 9.2.1 as it said in the GetInfo box, so all of those later updaters were being honest. What I really needed was the 9.1 updater found here https://support.apple.com/kb/DL1322?locale=en_US
Once I got 9.1 installed, I was able to bring it up to 9.4.3, which will work just fine.