I would like to know everybody's view on the timeline changes that occurred in this two-parter. Here I don't mean the actual temporal mechanics, as those were already explained by Christopher.
Now, the story is quite straightforward
Sisko and co. arrive in the past -> there is a muck up and Bell dies -> timeline leads to a devastated Earth -> Sisko and Dax save the day -> everybody goes home aka Happy ending
So it would appear that there are 3 timelines:
Timeline A - original timeline: Bell saves the day
Timeline B - destroyed timeline: Bell dead, Earth destroyed, Romulans at Alpha Centauri
Timeline C - corrected timeline: Sisko saves the day, pretty much like the original one
But here is the problem: we as objective, external observers have seen that as Bell is killed Timeline B is created. BUT lets look on who corrects the timeline, is it Kira and O'Brian? No, the same 3 people who were still in the past correct the timeline by themselves.
Do you see the problem? If the same people who make the mistake and created a different timeline repair it still in the past, why was a different timeline created to begin with?
Let me modify The City on the Edge of Forever to follow in the same lines as Past Tense:
On Gateway/Time planet Kirk and co see McCoy jump into the portal. they are then unable to contact Enterprise so they follow McCoy into the past.
In the past McCoy saves Keeler, lives with her for some days and is inadvertently saved by her.
As she is dying McCoy is reunited with Kirk and Spock, who arrived in the past just some minutes ago.
All three return to the future.
You see my point?
Now I think there are 2 possibilities:
There are only 2 timelines:
Timeline Alpha - original timeline
Timeline Beta - timeline with the predestination paradox
I believe that this set-up would still follow Christopher's temporal rules.
Here in Timeline Alpha is the one where Bell is alive, but as he would not have the right contacts to media would in the end effect lead to the same future shown in Timeline B.
Timeline Beta: Bell's death forces Sisko to take his place and actually lead the Bell Riots to a favorable end, WWIII and UFP.
The problem here is that Sisko studied the history. He ought to have stumbled over the Bell/Sisko picture, therefore he should have been forewarned, BUT he was not.
Here the 2. time-travelers actually correct the timeline.
The 3 timelines set-up is correct, but it is not Bell's death that creates Timeline B, but it is Sisko, Bashir and Dax living in the 21st century post-Bell Riots.
I don't know what changes they could make: Dex not keeping shut to the media; discovering that "Bell" is still alive could put him on trial and roll back the better conditions introduced post-Riots...
Well, what do you think?