CEL came back on after about a week, so I decided to take the plunge and change out the IMRC actuator, since I had a spare one of those too.
Once I got it changed out, I could tell immediately that the arm on the old part was retracting very slowly compared to the new one, once vacuum was applied. So while it was in a sense still working, it wasn't working effectively. And I guess not knowing how that part was supposed to look while it was operational had me tricked into thinking all was OK with it.
So I think all should be OK now, and I don't expect to see the CEL again now for a good long while. At least not for this issue.