Why not just send more cubes then? From what I've seen of the Borg, even if they can't match the Federation in terms of sheer numbers (which I'm sure they can), each individual cube is worth a lot of regular Starfleet ships. Half a dozen cubes would have stomped all over Starfleet's forces sent to stop them, so what was the problem?
It also raises the question of why the Borg sphere had time-travel technology in the first place, and why this wasn't attempted again later. We know that the Borg survived the loss of the Queen (ie, the whole of Voyager until, perhaps, Endgame), so it doesn't make sense that they'd just try time travel once and then give up. For example:
1. First attempt to time travel and assimilate fails.
2a. The Borg launch a second attempt from with the Delta Quadrant. Federation doesn't know of this because, well, they're not around to see the attempt, as the Ent-E was.
2b. The Borg just send a few dozen cubes at the Federation and simply overwhelm them. Or hundreds even, if they don't want to just nuke Earth into oblivion and would instead prefer to expand into the Alpha Quadrant from Earth.
The borg had assimilated enough humans and starfleet technology where they assessed that it wasn't worth the effort. From their point of view, to be efficient as possible it's better to just go back in time and get rid of them so they will no longer continue to resist.
And how would they judge where and when to pop out of the portal? not even assimilating a native would tell them anything mmore then what we know now they could and likely would come out at an incorect time and get curbstomped by the natives. Even if they came out at the corect time it is also likely that they would get blasted by their predecessors and contemperaries.
Trial and error. Each sphere can pick early points in history and the next sphere can go to a different point in history after the previous one and see if they conquered the galaxy yet. If they did not make any progress then they'll go back further in time to the previous one and try to stop the source of the resistance.
That is not what I was talking about. You are presuming either that the Borg have access to an infinite supply of energy, or else that time travel has no cost in terms of energy, since your idea requires that the Borg have essentially unlimited range in terms of their time travel. This is an unjustified assumption. Try to think a little before opening your lips, please.
They wouldn't need an unlimited supply of energy to go back in time at most a few thousand times. And even if this isn't true some how, again, we see time-travel done all the time on star trek and your objections don't stop this from happening therefore it's not considered a problem in the star trek universe. Heck there's even some cases where they go back in time in the star wars too. It doesn't require much thought to debunk you. Maybe you should take your own adise and think before you talk yourself.