Unless I missed something key during Descent 1/2, how is it that the Borg are once again a collective?
Only a small (very, very small) group of Borg separated from the Collective and regained individuality.
The Collective itself was never broken.
There are a couple of canon quotes from that episode that are worth mentioning;
LORE: No, you did that. You and your friends. All I did was clean up the mess you made when that Borg you befriended returned to his ship.
DATA: Hugh interfaced with the others and transferred his sense of individuality to them. It nearly destroyed them.
LORE: When I stumbled on their ship, they were lost, disoriented, they had no idea how to function as individuals. They couldn't even navigate their own vessel. They had lost their sense of purpose. I gave them their purpose. And they gave me mine.
PICARD: What will you do now?
HUGH: I don't know. We can't go back to the Borg Collective, and we no longer have a leader here.
PICARD: I'm not sure that's true.
HUGH: Perhaps in time, we will learn to function as individuals and work together as a group.
As far as "First Contact" is concerned, there's no evidence to show that Hugh's small group of ex-Borg drones (probably numbering less than a few hundred) had any further influence on the trillions of drones that made up the greater Borg Collective, any more than did the ex-Borg drones seen in the Voyager episode "Unity" who also experienced a similar awakening.