The ship was a single ship alone in hostile territory, and Neelix was the only member of the ship with a remotely diplomatic skillset. He should be on the bridge when they first meet a new race, and he should be involved in top level discussions about how to deal with them.
I think the Borg like to play the long game. They're not playing to win the game as fast as possible, they're just playing to win the game. So they play salami. Attack one planet, once they've completely dominated it, move on to the next.
But that doesn't explain why they don't send more than one cube at a time to Earth.