Seven knows about it because of the signal sent at the end of "Relativity" - that's the whole reason that bit was included, to fix the continuity error of her knowing about the Borg being present at First Contact. Otherwise there is no way she could have had that knowledge.
And as I said, the Pogo Paradox is attempted interference to prevent an event being the catalyst of the event they were trying to stop. The Borg caused the change in Earth's past, making the Enterprise follow and undo it. That sequence of events is what "always" happened (in-universe) in Treks timeline. A loop involvong a change and an undo.
Don't get me wrong, it makes very little sense as does all sci-fi time travel. But I take the in-universe explanation from the guys in the future (+ENT's out-of-universe retcons) over the idea Enterprise is an altered history, which renders it moot as a prequel and ruins it's tie-ins to the later series' and movies.