If Sisko wasn't always Gabriel Bell, then why do the crew return to the same future they left?
That's what I've been trying to tell you. They
don't return to the same future they left. They return to a future that's 99.9999999% like the future they left.
The events of the episode are exactly like what happens in
Back to the Future:
1. In the original timeline, Marty's dad is a weakling pushover to his boss Biff. His mom is overweight and unhappy with her life. His brother and sister have shitty jobs. In the original DS9 timeline, Gabriel Bell was responsible for the Bell Riots.
2. Marty goes back in time and inadvertently changes history by interfering in how his parents originally met. Sisko and Bashir inadvertently change history by attracting Bell's attention and accidentally getting him killed.
3. While not instantaneous, Marty's actions change future history by causing him and his siblings to not be born. In DS9, history is immediately changed from the Defiant's crew's perspective.
4. Marty carries out a plan to get his parents back together in order to "right" the timeline, although the circumstances behind that are now different. Sisko assumes Bell's identity in order to "right" the timeline.
5. When Marty returns to 1985, despite his actions to "right" the timeline, things have changed for him and his family (for the better.) But the rest of the world at large seems to be no different than it was before. When the Defiant crew returns to the 24th century, history now has a photo of Sisko as Bell, even though the rest of the world (or universe) at large seems to be no different than it was before.
Plus, if someone in the 21st century was able to take a photo of Sisko and have it survive to the 24th century, then it stands to reason that that same photo of the real Bell was taken in the original timeline, and Sisko et. al would have recognized him. It's not like anyone said, "well, no one actually knows what Bell looked like, so we wouldn't be changing history by having you impersonate him."
Even the
slightest deviation from the original history would result in a vastly different future. If you assume Sisko wasn't always supposed to be Bell, he could have replicated the original Bell's actions and words down to the slightest detail and it would STILL make things turn out differently. We've heard of the butterfly effect, haven't we?
Sorry, that's not how things work in the fictional universe of Star Trek (at least until nuTrek.) Take the Mirror Universe for example. Based on what we saw in ENT, the earliest that the two universes diverged was in the 20th century (it could have been even earlier, but that's not really important for this example.) So by the 22nd-23rd-24th centuries, as you say, things should have diverged so much that there would be no way that someone in the prime universe at that time would have an exact duplicate in the MU. But they all have duplicates. It makes for good storytelling, but the logic behind the idea is faulty.