I don't usually get into brain-twisting time travel discussions, but reading these interesting posts has nevertheless got me thinking...
On the surface, it seems to me that there's no reason Sela doesn't exist in the "regular" time line. The crew had not met her at the time of this episode, but she could easily have been around...just not known.
Think about it this way: in Time's Arrow
, Data's head had been discovered on a cave on Earth. Following the events of that story, the reason became clear as to how it got there and when. Couldn't Yesterday's Enterprise
be similar, minus the awareness of Sela/Data's head?