As far as reel 75, it seems to match up visually with the AB clip, but the description is pretty nonspecific, really just Brian playing the piano and it being dark (not unusual in a studio), which certainly could be a completely different time. And if the Inside Pop reels were the source, shouldn't they have the audio with the film - it's printed on the film, isn't it?
Could the "Do Wa Wa" vocals be backing vocals for the "Reconnected telephone direct dialing" lyrics of Cabinessence? Of course we don't have any corresponding music tracking for that section. I've been thinking lately that those lyrics were intended to go in the Who Ran the Iron Horse section like the "Truck Driving Man" lyrics, Telephone in the first chorus and Truck Driving in the second. When Carl and Dennis worked on the song in 1968 they either intentionally left out those lyrics from the first chorus (which was a good move IMO) or they forgot/didn't know about them.
The most depressing news from the time when all of this info first appeared was that the actual film and the audio tracks for the film could have been recorded on separate reels of tape, and the audio and video were not archived together or printed on the same strip, according to the way remote recordings were done in those days among news organizations and the like. So I think the odds are against having one strip of film with both video and an audio track. Which, to me, is devastating. I think they only synch'ed the two up for the cuts which were going to make the final edits. I could be wrong.
I think the "reconnected telephone" idea is a solid possibility - something attempted or planned that either got scrapped or remained unheard as the work progressed. We have no way of proving much beyond those notes, but it would make sense to have a Cabinessence related session on the Cabinessence reel of film.