Finally got it to work.
Moving the remote closer to the PLM did not help, I had tried that many times before. Same for the factory reset.
Then I got the idea to move it closer to some other dual band devices. First near a back of three recently purchased SwitchLincs that are on another AC phase, only five feet from the PLM but on the other side of a common wall. Still no joy.
Then I thought to try something on the same AC phase as the PLM. I tried a brand new on/off module that I had purchased for the Christmas tree, but hadn't used. Plugged it in, set the remote to linking mode, and put it down about five feet from the module. Presto, the ISY was able to link to it.
Once linked, the on/off module is not needed. The ISY sees button presses on the remote and can respond with complex commands as I had wanted.
I'm guessing that for some reason the remote was having problems seeing the engine query. Could be a weak receiver on the remote or some interference or a combination. The ISY has absolutely no problem sense key presses on the remote in any room in my house. So what the communication problem was, it was asymmetrical.
...Fred