I'm no recording expert, but this is my take on your question ....
The source material WAS the same for both the stereo and mono versions - it was the original 8-track master recordings.
The differences between the two mixes stem from the fact that the mix is a "live" process - you play the 8-track master tape, and make changes on the mixing desk as the track is playing. You may want the vocal pushed up in one section, or the stereo panning altered for one part, or whatever.
The results are recorded on a new tape and this becomes the master to be sent to the presses to create the records.
For a mono mix you don't use any panning - everything goes into the centre of the track. You have to create a totally separate mix for the mono and the stereo as the balance of instruments/vocals etc will have to be different from one to the other.
So the stereo and mono mixes are done at separate times - in most cases they end up being fairly similar to each other, but in some cases (like the end of Helter Skelter etc) they end up with differences.
That may have been done on purpose, or simply because they had forgotten how the mix had gone from one version to the other.
I'm sure there are better people on this forum to answer question, so I will leave it to them to correct my mistakes
I agree with all your points. So, it really gets down to ... what's missing in this picture sort of questions ...
Say the difference in the length of Helter Skelter was ... artistic whimsey.
Say, the difference in the speed of mono and stereo versions of "Don't Pass Me By" was ....simply the engineers forgetting what was previously decided
But, the fiddle ending to "Don't Pass Me By" is not simply more of the same, it's different. Different take (or portion of a take) and different in that one version has an overlap with the music before it and one does not. So, one of the question becomes ... more succinctly then I put it in the Thread title ... when is the pre-mix master the pre-mix master?
One could suggest that - as far as the Beatles were concerned during White Album sessions ... the answer was never or
- quite possibly - I am missing key knowledge of the process.
One could counter that the Beatles simply could not get with the program and make the pre-mix master the pre-mix master and get on with life. They couldn't (for whatever reason) control their artistic nature and needed to continually mess with things. That is quite possible. Except it seems at odds with so much we know. Mark Lewisohn details rather long mixing sessions after long recording/mastering sessions. Reportedly, the White Album sessions were so awful for the group and those involved that no one wanted to be there and engineer Geoff Emerick outright quit. Yet, seemingly in contradiction to all of that , we have (at least) some of the Beatles in remixing sessions for the first time for BOTH stereo and mono versions and
taking hours upon hours for the effort ... because why beyond more people were listening to stereo players, thus it deserve more focus?
One would think that after fights about what to put on the album, fights about what take(s) to use for the pre-mix masters, fights about what elements on the 8 track to bring forward or keep in the background ... after all that they had nothing better to do then revisit pre-mix mastering decisions? I always thought the Beatles were pretty smart lads ... so it make me wonder ... what don't I understand about all of this?