Re: Timeline branching going forward *spoilers*
More to the point, we have no idea whether the events of "Relativity" are themselves occurring in an alternate timeline because it's only discussed subjectively.
In other words, while they exist in a timeline in which the Borg interfered with "First Contact", we don't know whether it was the Borg of their timeline that interfered or whether it was Borg from a different timeline that did so. And if you believe that when you travel into the past you jump tracks, then the latter must be the case.
It was the best of Trek, it was the worst of Trek...
"If I lean over, I leave myself open to wedgies, wet willies, or even the dreaded Rear Admiral!"