So what does merging entail then? Does everything happen in both timelines? So when she goes back to her present and time merges are there 2 of everyone?
If not and she ends up in her present with the changes from universe 2 in effect then I think a simpler solution would be to say that she leaves universe 1, arrives in the past and makes a change, and then she is now on a new branch called universe 2. Then when she travels forward in what is now universe 2 she isn't affected by any of the changes because she was originally from universe 1.
Wow I really like that concept Drew! hmm.
Time lines merge on a single track. There is still one of everybody unless you're in limbo. For the story we have going right now everyone has to stay on the same "time track" parallel universe aren't incorporated in the story (yet). So the science works when you travel back to the your present the time tracks merge unifying all history from the changes moving forward.
We are making it so that there is a way to prevent paradox. Weather it is explain in the story like I describe above is another way, it makes is so when a character says "I'm going to change the past." When the time traveler goes to the past to change the future nothing changes until she comes back.
When she come back, everything from the point she disrupted is replaced with a new set of events.
Referring back to staying in universe 2. I'm accommodating for that later so that can happen. If anything it's science fiction, it just kinda has to make sense. That's for that suggestion though. Why doesn't she just travel back to universe 2 to her "new present" instead of 1?
Well, to answer that question we don't know. It's the theory of time travel. Traveling forward in a parallel universe will probably be a different technology.
Limbo will create all kinds of complications. how does limbo work, what happens when you stay in limbo? what can you do in limbo? hmm.. more thoughts.