If you really want to make things complicated, try reconciling Destiny and First Contact.
Never bothered me any. I just assumed that that just meant that in the Borg-altered timeline seen in
First Contact, the Borg who reached 2063 Earth continued to exist because they'd originated in an "alternate" timeline (the same way, for instance, that Future!Janeway didn't cease to exist upon changing her own past in such a manner that should have ensured she'd never exist), assimilated 21st Century Earth, tried to contact the rest of the Collective in the Delta Quadrant, and then realized, "Hey, that's weird, there are no more Borg anywhere else in the galaxy."
So in this altered timeline, from everyone else's perspective, Earth and the Alpha Quadrant are the origins of the Borg, rather than Arehaz in the Delta Quadrant.
If so, wouldn't the best outcome have been for the Enterprise to defeat the future Borg but have Cochrane's experiment fail (temporarily), thus retroactively destroying the Borg at the cost of delaying the Federation's birth?
What makes you think the Federation would have ever formed, at all, had United Earth not been founded?
The Federation's job -- be it Starfleet or DTI -- is to preserve
their history, not someone else's potential alternate better history, nor to protect some "uptime" faction's history. Just theirs, period. And
certainly it's the obligation of a Federation Starfleet officer to preserve the Federation first and foremost.
ETA:
Oh, sure. I'm not criticizing Engines of Destiny (having never read it), I'm just saying it makes more sense to assume that the Borg conquered the Alpha Quadrant in 2373 than to assume they then traveled back to 2063 from 2373 (since there would have been no Picard at the Battle of Sector 001 to detect the weak point in the cube and direct the rest of the fleet to attack that point).
Maybe someone else would have taken on Picard's role as Locutus, etc. etc. since the most probable events had to happen.
Well, if the premise behind
Engines of Destiny is that Kirk's rescue from the Nexus in 2369 prevented him from saving Picard in 2371, then Picard would have still been Locutus in 2366-2367. He just would have died in 2371, thereby preventing him from defeating the cube in Earth orbit in 2373 and then following the sphere back to counter its attempts to change the events of 2063.
ETA:
If you really want to make things complicated, try reconciling Destiny and First Contact. If the future Borg assimilate the Earth before the Columbia contacts the Caeliar, doesn't that prevent the Collective from forming in the first place? Do the Borg in the Delta Quadrant get erased from history? If so, wouldn't the best outcome have been for the Enterprise to defeat the future Borg but have Cochrane's experiment fail (temporarily), thus retroactively destroying the Borg at the cost of delaying the Federation's birth?
The Borg
had to travel back in time to 2063. It was a predestination paradox--they knew that their interference in Earth's history would lead to their eventual creation.
Why's that? For one thing,
Star Trek: Destiny doesn't give us any indication that the Collective had any memory of their own origin. In fact, the very fact that they adopted for themselves the name "Borg," out of the broken last thought of a Human being "assimilated" by the degraded remains of Sedin's consciousness, strongly implies that that initial "assimilation" process caused the early Collective to
lose any information it had on its own pre-Collective past. After all, why would they only use the fragment of the last word of the last thought of a dying man for themselves otherwise?
Further, there's no particular reason to think that the Borg's actions in
Star Trek: First Contact led to the creation of the Federation. Sure, it's
possible that Riker's and Co.'s example led Cochrane to abandon his cynicism in favor of idealism (presumably thereby becoming a leader in Earth's planetary unification movement, thereby becoming one of the "Founding Fathers" of United Earth, which of course later leads the way towards creating the Federation), but it's just as possible that he would have embraced such idealism for the reasons history records -- the eye-opening gravity of encountering extraterrestrial life for the first time.
So really, First Contact was a big setup. The Borg sent a cube to Sector 001 and baited Picard into showing up by sending him crazy Locutus dreams. When the Enterprise arrived, Picard thought he was destroying the Borg cube with his inside knowledge--but the Borg ship deliberately self-destructed in order to get on with sending the sphere into Earth's past.
Makes no sense.
If their goal was to send a ship to the past to bait the
Enterprise into chasing them back so they'd inspire Cochrane to create United Earth (and thereby U.E. would send out Hernandez aboard the NX-02, leading to the creation of the Borg), it would make more sense for the Collective to send the sphere into the past
while the battle is still raging, thereby luring the
Enterprise away from the cube to "restore history" and leaving the cube behind to finish defeating Starfleet and assimilating Earth in 2373.
The Borg were aware that the Enterprise would follow due to their experience with time anomalies, and they fought the Federation ship with one hand tied behind their backs once they were in the past--because they had to lose.
What was with the attempt to take over the
Enterprise, then? Such a plan would only require an initial strike against Cochrane on their part; it would be much more logical and efficient for them to simply allow the
Enterprise to destroy their sphere and make no attempt to assimilate it, thereby allowing the
Enterprise crew to inspire Cochrane with no Borgy distractions aboard ship.
* * *
Christopher suggests in
Watching the Clock that the Borg in
First Contact traveled to 2063 after an uptime faction in the Temporal Cold War thwarted other uptime factions' attempts to prevent the Collective from gaining time travel technology. That's possible, but still leaves open the question of why the Collective tried to change Earth's history in the first place.
Personally, I'm inclined to assume that the Borg are as loathe to use time travel as any other 24th Century faction, for fear of losing access to the advanced technologies created by the unassimilated cultures upon whom the Collective relies for its own technological progression, and that they only use time travel techniques like the one in
Star Trek: First Contact as a way to test the ingenuity of cultures whom they find especially resistant (a hypothesis offered in
Mission Gamma, Book Four: Lesser Evil).
Heck, maybe the Borg had every intention of un-doing their own interference if they discovered the Federation wasn't able to un-do it themselves. That might go a long way towards explaining how Seven of Nine (and, therefore, presumably the rest of the Collective before she was separated from them) knew about the
Enterprise-E's role in thwarting their plan in VOY's "Year of Hell."