I'm still not getting it.
As of 2024, both the Federation and Confederation timelines are possible futures. Therefore, characters from either timeline should be able to travel back to a point before the divergence - and even meet each other. I see no reason why this shouldn't be able to happen. For example, even though Picard and crew are now from the Confederation timeline, they will still meet Sisko and the DS9 crew if they stick around long enough, because the Federation timeline is still a possible future.
Also, the Guinan from "Watcher" should still have met Picard in the 1800's, because again, the Federation is still a possible future.
The divergence has not happened yet, so logically speaking, one timeline shouldn't be "dominant" over another. There ARE no separate timelines at this point.
Two timelines cannot exist at the same time. Here in 2024, on April 15th, Q (or Picard) did/didn't do something that leads to the Federation or the Confederation. The PIC crew that came from 2402 is from the Confederation "choice". Q's interference made the Confederation scenario THE timeline. Picard (, Data et al) never went to the 1890's to meet Guinan nor Twain. So, Guinan never met Picard. Other time travel (episodes/movies) like TVH or Assignment Earth (which may be important here), or the other ones may have still happened, but for different reasons.