On the first point: perhaps they've destroyed the equivalent of a cell phone tower, and what's coming is just the repairman. Or perhaps it was just a monument to General Gug, conqueror of this dead, lifeless rock.

On the second point: over a billion years, the race in question may have gone extinct, or the loss of signal may not be noticed because its communication mode is as obsolete as smoke signals. Or it may not longer be listened to because the alien-monitoring budget was slashed in a cost-cutting reorganization back in the Cambrian period.

Bruce