Okay, but here's the real problem.
If the Borg want to assimilate the Feds, why just send one Cube? Why go to all the trouble to time travel a la First Contact? Why try and formulate some kind of plot like Dark Frontier?
All they'd really need to do is gather up a few thousand Cubes, transwarp to Earth (or use a hub) and get busy, honestly. Maybe a few million, if they really want to make sure they'll clean house.
You've got to have a little suspension of disbelief with these guys, both in TNG and