Thanks for that takedown of VOY: Relativity, Romulus Prime. I have been saying the same damn thing for years. The writers of VOY thought they were being clever when they added that nod to First Contact by Seven of Nine's admission. It's bogus. The Federation only owes the Borg their advancement it weapons ans ships since their introduction in TNG: Q Who showed how outclassed the Galaxy class ship was.
Seven Of Nine states that the events of First Contact were predestination/casualty loop, where the prevention of events acutually causes them to happen. If you think about it 7 of 9's admission makes no sense. The Borg travel back in time and succeed in conquering Earth, FROM A 24th CENTURY POV. The Enterprise-E was built in the 24th Century by the Federation. So how can it possibly exist in a future where the Federation never happend? The Borg changed history, the ENT-E trapped in the temporal wake (displaced in time which is how they weren't affected) followed the Borg back and made sure history would happen similar to they way it was recorded by the 24th century.
That's not a casualty loop. The loop can't be the Borg travel back in time and succeed in their goals in the 21st century (which the ENT-E crew and we the audience saw from a 24th century pov), and that the Borg would fail in the 21st century and not assimilate Earth. And yet somehow 7 of 9 knows the realities of both of these futures? BS. VOY writers tried to be clever and failed at it.
If anything First Contact shows us string theory time travel. Linear time travel that affects what the future was and negates it making a new future. Star Trek 09 is chaos theory or rather butterfly effect time travel. Changing events of established history which develops a universe vastly different from the one the travelers originated from. Under chaos theory the travelers timeline will continue to go on since butterfly effect is an alternate timeline of events. String theory First Contact was done under linear time travel so the future was in real jeopardy of being rewritten and extinguished.
Yesterday's Enterprise and Sela would be Chaos Theory/ Butterfly effect. Only difference is an act that should of happened didn't because of a hole in the universe, which Picard fixed by sending the ENT-C to face it's destiny and be recorded in history.
Re-read that quote. A pogo paradox, NOT a predestination one! Where the first changes, made to alter history, lead to the second changes that (re)create the original timeline. It's a loop that includes the creation of another timeline which is then (possibly? presumably?) erased.
As for the Borg, the only was to make sense of the repeated retcons is to say the Borg were already coming as a result of FC and "Regeneration" and Q simply tipped us off to the long forgotten threat in "Q Who"
As for what Seven knows, the reason she knew about FC was because of the signal sent by the survivors at the end of "Regeneration" (which in that way is a neat bit of fix fic)