Here's what I think about Tosk: Sisko isn't violating the Prime Directive by allowing O'Brien to help Tosk. He's upholding
the PD. The hunt would have gone uninterrupted had none of the DS9 crew befriended Tosk. So by setting the hunt back on, Sisko and O'Brien are putting things back the way they were before Tosk ever came aboard DS9. That's not interference.
Vulcan Logician wrote:
Was Gabriel Bell ever Gabriel Bell? Probably so, because Sisko would have seen himself as Mr. Bell having studied him in the Academy.
Who's to say he didn't? Sisko may have looked at "himself" and said, wow, that guy looks like me, but otherwise not thought anything of it. The point is, we have absolutely no idea if the 'original' Bell that gets killed in the episode really is the original. For all we know, Sisko was always Bell.
And since the characters returned to the same future they left, I find this likely.