To quote O'Brien "I hate temporal mechanics"
"Regeneration" is one of those things that creates a paradox that cannot really be untangled. The real question is, were the attacks along the Neutral Zone a random scout and recon mission by the Collective, or was it a result of the signal sent by the drones in "Regeneration"?
If the first is true, then it was Q who screwed up the timeline by introducing Picard to the Borg earlier than what history had planned. Had "Q Who" not occurred, the Federation (and the Romulans) would have not learned who destroyed their outposts in "The Neutral Zone" and the whole Alpha Quadrant would have been taken unaware by the Borg. There would have been no prep time noted in "BOBS", and no Defiant class. The Borg (or at least their Queen) wouldn't have singled out Picard who was used to defeat them during their first really attack on Earth. Our side of the galaxy would have probably fell pretty easily.
If the second is true, then it was the Borg themselves who understood the complexities of the timeline (which the Queen and Seven of Nine both imply) and, realizing their second attack upon Earth was going to fail, "reset" the timeline into a loop by launching the sphere and giving them endless opportunities to successfully assmilate Earth. The Borg do not seem to regard a temporal directive. Being able to open temporal vortexes gives them a million changes to take the Alpha Quadrant.