TrekCore shows us where the story came from: the earlier version of the script available there shows the writers' desire to distribute the heroes evenly through the various locations, of which the Montana missile silo farm and village is very important to setting the mood of the story.
As can be expected, the writers would initially fumble such a three-ring circus, having characters inexplicably appear in the wrong location or fail to contribute when supposedly available (Crusher is all over the scenery, say). That would be sorted out in the final version. But drama-wise and story-wise, it would be extremely important to get Riker and Troi away from Picard, so that Sloane would have the chance to act as the skipper's conscience. And stranding both of them on the planet without them even realizing they have been stranded was a pretty smooth way of doing that.
I think it just adds to the darkness of the Borg half of the story that half the heroes are ignorant of the grim struggle taking place above their heads... It's also fun to see how Picard is ultimately surrounded by spineless yeasayers: Crusher and Worf both meet the bill perfectly, despite superficially being the characters the most likely to talk back.
It's just too bad that we never learn what happened to Guinan, as again some superficial factors like skin color and gender conspire to remind us that her character is being replaced by a "cheap copy" here - another character who serves in the role of listener and arguer, but without further fleshing-out or padding elements to her (vs. Guinan's previous history with our heroes, her admittedly somewhat silly mysteriousness, her considerable age, yadda yadda).
But no, I don't see the time travel plot working without the major contribution from the Zephram Cochrane half of it. Without the Cochrane story, and the splitting of our heroes between that one and the Borg fight, the time travel would make even less sense and would feel completely tacked on. Yet a generic fight with the Borg would be too simple a story to entertain us.