Understanding timelines of these people in the Yesterday movie

You've described the premise of the movie: This is an alternate timeline where John, Paul, Ringo and George did not create a successful band and write songs. However, within the movie at least 3 people remember the original timeline (in which The Beatles wrote the songs, and John was later killed).

Although this is speculative on my part, it is my sense by the way that John conducted himself with Jack that he must have been apprised of the situation somehow, presumably by the pair besides Jack who knew of the music. And I think John, who reasonably is still as talented (iirc, he also painted and we see that he does that in the new timeline), accepted that there was a life in which he became a musically successful because he no doubt wrote or considered writing songs.

That is what he was talking about to Jack -- he understood that he could have taken another path but was content with the one he chose. IMHO, truly a profound scene.

Note that John did not act completely baffled -- perhaps too, if anyone else had perception of both timelines, it was John. In fact: Perhaps the idea is that John understood that his decision had caused the bifurcation of reality -- Jack was just someone who somehow drifted from one reality to the other but John (and rightly so) was the true agent of cause, just as he and the other Beatles, whom he could have met in passing, were the actual authors of the music.

We've got to accept that Richard Curtis was not too worried about a consistent alternate timeline - after all, Ellie fell for Jack when he sang Wonderwall, a song she could not possibly remember after the world forgot Oasis.

The divergence point is pre-Beatles - it sweeps up not just the word "cigarettes" (a name borrowed from the French town in 1830) but possibly tobacco itself - Rocky balked at "cigarettes" but didn't seem to recognise the idea of "smoking", and I didn't spot anyone else smoking at any point in the movie, suggesting that tobacco never crossed over from native American culture to their white invaders.

Although colas exist, the original (Coca-Cola, 1886) which contained cocaine, does not.

Perhaps this is really a more sober timeline with fewer narcotics?