Why didn't the batarians notice a massive asteroid heading for a relay?
The Bahak system is not heavily populated, so it's possible that there wouldn't be any observatories or space stations, or even nearby ships that would observe any objects any more than a few AU away from the planet. Of course there's no information about satellites around Aratoht, since the Hegemony keeps that stuff secret anyway.
Thinking about it, the bigger problem with some of the logic in Arrival is that some batarians are fully aware of the plot to destroy the relay. Even if any claims from Kenson are met with incredulity, it would make sense to send a ship to scout the relay to see what's going on.
I don't know but maybe they should have.
Having to fight through Batarian special forces desperately trying to save their world from what they see as a brutal terrorist attack instead of just conveniently indoctrinated Project staff that can be gunned down guilt free would be an improvement.
Well, we do fight through batarian soldiers when freeing Kenson. Beyond that, there wouldn't be any reason to encounter any of the free population of Aratoht, which is about 90,000 people.
Speaking of which, I wonder how many of the batarians' slaves would have been saved along with those 90,000 had they been successfully warned.