When Mrs. Vikor hands David a gun and he opens it to see if it is loaded, one can see that three of the bullets have been used, as they have the firing pin mark on them.
At around 25 minutes, when David is fighting the alien guard in the stairway to the regeneration booth room, as he tosses the guard's body on the stairs leading down, one can see the mattress the guard falls onto.
Nexus has an agent listening (and taping) in the Vikor house and hears where Vincent is and immediately tells Nexus. If he's paying this much attention to the bug in real time then he should have informed Nexus and George Vikor earlier when Vincent explained about the aliens to Mrs. Vikor. And Nexus should have been waiting for him at the plant earlier since they would have heard the plan of Mrs. Vikor and Vincent .
Mr. Vincent did not shut off the gas from the fireplace - the first thing that should be done if the gas has been left on.