"Back in 1991 I went to a one time only theatrical showing in New York City of all 5 Trek films (up to that date). They presented the films back to back. They called it "Sit Long and Prosper". The showing of ST:TMP was in 70mm, and had most of the extra footage that is included on the extended pan & scan VHS, but not all of it.
There were never any 70mm prints made of TMP, only 35m prints. Some of the 35mm prints had the extra TV footage spliced in for the "Sit Long and Prosper" showings. Even the print that was pulled to show Robert Wise at the Director's Guild in preparation for The Director's Edition project was one that had these scenes spliced back in. That print was also missing the overture.
70mm prints were planned initially, but the frenzied post production period didn't allow for them to happen. A film has to be locked in order to produce the 70mm version because the sound has to be recorded to the mag track. That may even have to happen in real time. As we all know, there was barely any time to get the 35mm prints made.