Ken is shown driving two Corvettes. The initial front end shot shows a C3 (1968-1982) model, while the following shots show a C2 (1963-67) model.
Catching a cold by sleeping with the window open on a cold night is an urban myth. Colds are caused by a virus. While someone would be chilled in the situation shown, they would recover once warmed up.
In the front view of Ken's Corvette, the front license plate was digitally blurred. Since there were at least two Corvettes used for this scene, this was likely a car borrowed from a crew member or sourced locally that did not have a movie license plate- plates that are set aside by the DMV for movie making. The plate was blurred to protect the personal information of the owner.
Ken arrived at the dinner in his Corvette and Amy was inside the entire time. She had no time to sabotage the brakes on Ken's car. Additionally, to access the brake lines and hoses on a low-slung car such as a Corvette would require a jack or a lift to get underneath it.
Anna suspects Amy is dosing her drinks. She was cognizant enough to tell Charles of her suspicions, and she was semi-ambulatory. Due to her suspicions, she would not drink anything else Amy gave her. Yet in the closing minutes, she is semi-comatose as Amy prepares to inject her as if she had continued to consume the drinks.
In typical Hollywood movie style, the brakes have failed on a car and the driver does not simply downshift and turn off the ignition. Even at roadway speeds of 50-60 MPH, there was enough open road shown in front of Ken's car to where he could have cut off the engine and used the engine as a brake to slow the car. He is even heard accelerating as another car approaches. Additionally, since he left from a residential area where he would have had to use the brakes far more often than on an open road, the brakes would fail long before reaching the open roadway shown.
Charles having been told by Anna of her suspicions of Amy drugging her as well as the totality of the circumstances with Christine's illness leaves Amy alone with both of them.
Amy hits Marilyn in the head with a wine bottle once and kills her, yet she hits Christine in the head twice with a heavy iron skillet twice and she survives.