The biggest plothole being, if they had time travel technology, then why not go back in time FIRST and THEN travel to the Alpha Quadrant?
They didn't even have to go back very far, just to any time before "Q Who?" was set.
Remember that the Borg are not really threatened by the Federation.
They probably wanted to see how their cube would fair against SF second time around, and should it fail, send a smaller ship to the past from the AQ.
I know I know ... if the cube was again destroyed, the Collective could have sent a vessel from the DQ to the past and again travel to the AQ.
Makes no sense I know ... but it's possible the Borg were aware of the fact that they should send a signal to themselves in the past (NX-01 Enterprise events) thus completing a temporal loop which would result in the present time-line while not really knowing just how would any other course of action fair.