Wouldn't dialogue confirm that one ship survived.
No, there is no dialogue about surviving ships. Anything to that end is conjecture from dialogue, not confirmation.
In "Best of Best Worlds Part II," Admiral Hanson says that they have assembled a fleet of 40 ships at Wolf 359. Then in "The Drumhead" retired Admiral Satie says that 39 ships were lost along with over 11,000 lives. That would be canon that 1 ship survived, whether or not it was salvageable is another story.
Not really. Hanson said he was mobilizing far more than 40 ships, so 39 lost might leave 156 survivors for all we know - but OTOH 40 is a round number and in fact only 39 ships may ever have taken part.
From what was shown in DS9's "Emissary," the battle probably did not take long.
Yet the Cube still lingered when the E-D arrived: following some "eddy currents" took our heroes to the Cube in no time flat, despite the Cube being able to outrun the E-D if it so desired.
And whatever personnel the Borg took for assimilation could be assimilated en route.
OTOH, as some of those people ended up in Delta Quadrant, the Borg may have lingered for the purposes of turning the shipwrecks into further Cubes, ENT "Regeneration" style, and then overseen their launch.
If so, the Borg would again aim at perfection, disabling and converting every single vessel within range. And they do go for perfection in several ways on several occasions, down to hunting down individual survivors in "Dark Frontier" (I guess Guinan best beware!).
So we have a Borg cube with enough drones it can carry and/or needs and 1 ship that posed no threat and 1 ship on the way and possibly Klingons on the way too, it makes sense for the Borg to leave that last ship and continue with its mission.
It never made sense for the Borg to engage those 39 ships to begin with. Why stop rather than just go to Earth and assimilate the planet? It's not as if the Borg couldn't have gone past Wolf 359. Or through the blockade without stopping, ST:FC style. Or much minded if 39 ships did their worst while assimilation of Earth was proceeding.
Stopping to fight was probably a Locutus decision - that is, Picard sneakily suggesting that assimilating some starships would be good for the Collective, thus delaying the assimilation of Earth. If so, though, stopping one ship short would be silly, when the Borg could have stopped 39 ships short just as well.
Once Drones were on Earth they would spread like a plague and even if the Cube were destroyed, Earth would be compromised and with Earth compromised, more Cubes could be sent.
Can the Drones really spread like a plague? Assimilation is far from simple: without transporters, the clumsy zombies could never reach their victims. And even the injection of nanoprobes does not create all-new Drones: some surgery is needed in addition to that. Cubes may be crucially important to the assimilation of crowds.
Those are my thoughts anyway.
And that's the great thing about the Borg: after decades of exposure, some pretty basic things about them still remain open to debate. Trek rarely does alien aliens, but these worked out just perfect!
Timo Saloniemi