Eliminating Earth as a factor could probably have better been accomplished by going farther back in time; gathering technology could have been achieved at a later date. But ensuring that the Federation is born could only be accomplished in 2063, because apparently Earth was a key ingredient in that, and Earth couldn't participate if not for the curious encounter with Vulcans at that date.
So perhaps the Queen just went back to ensure that Cochrane flies? Alone, the poor man might have achieved nothing. But send back people from the 24th century, and their "repairs" to the warp contraption would ensure that the thing gets warpborne - after a few iterations of time travel at least. Sacrificing one Cube and one Sphere and one body might have been a small price for that.
Second-guessing the Borg is something our heroes didn't have the time for. But it's something the audience certainly can do on their spare time...
As for Cochrane's achievements, I trust most of the work would have been done before the war, in which Cochrane apparently lost a great deal (in both senses of the expression, I guess). But Earth wasn't bombed back to stone age in the war, as evidenced in various episodes; Cochrane could still have had powerful backers. Why did he install passenger seats on his craft if not for observers from the funding parties?