The book itself says that since Enigma already exists that time travel attempts to kill it won't work because it exists outside time and space.
The Dominion showed up but when Stasis tried to upload himself into the husk (since they killed the brain) it was already possessed by Enigma.
Apparently the Dominion that sent Omega Sentinel back in time is still around but hasn't shown up.
"Cable was right!"
I believe the entity that sent Omega Sentinel back was the Trickster Titan, which is not a Dominion iirc