Unlike apparently most of the respondents on the thread though I can accept and give the answer I don't know and leave it at that. I had hoped somebody knew something I didn't and could point me the correct way.
For the sake of your certificate(s) and from some of the incorrect information given in this thread, I'd suggest writing a letter requesting an interpretation of "authorized instructor" for instrument training, instrument knowledge endorsement, and IPC endorsements, and getting the FAA's response back before attempting to do any of those three with a CFI-Airplane certificate.
The letter from FAA National Legal Counsel will stand up in the courts that count.