udev existed before systemd, but was merged to it sometime in 2014. By quick Googling there are still several forks that maintain it as a separate component.
My understanding is it's in the same repository (for convenience), but is logically separate, so "forking" it mainly means copying the correct directory.
Eh, not really. Many Parabola users who use OpenRC instead of systemd still use systemd-udev, as opposed to eudev or one of the other non-system forks.