Re: first contact lack of common sense?
You're right about it not making sense. In reality, the Borg only needed to send 50 cubes to guarantee the end of the Federation. Also, if the Borg wanted to time travel, why attract the Federation's attention doing it on Earth's doorstep? Cobra
Presumably Borg temporal sensors have a very short range, so they needed to get close to Earth before figure out their time travel calculations or something.
"STAR TREK is... Action - Adventure - Science Fiction."
-- Gene Roddenberry, 1964, top of the first page of his original pitch and outline for Star Trek