Your script should describe what the viewer is seeing and hearing. In many places you give background information that's not going to be available to someone seeing the movie. If you want to leak the information, do so visually first.
on p. 2 "Really -- who could sleep with the knowledge of their kid being missing?"
Nowhere is the length of his absence made clear. All we see are some scenes inside the house. You might have the camera float over articles thumbtacked to a bulletin board detailing the boy's disappearance to give context.
Also, I would not put in a telephone number. There will be those people who will call the number. It will be maddening for the person who may have that number and could lead to potential legal action. I would simply state "... call the Lisle Police Department."
"In the situation Richard’s in, how can you blame him for not wanting to listen to this garbage? And why the hell is Joan worrying about this if her son just went missing?"
As Directorik said, as the writer/director it helps you. For the viewer this means nothing. The script should evoke these reactions without actually being stated, since there is no mention of how long Adrian has been missing. And you mention this at the top of the page before his disappearance is even mentioned at the bottom of the page. In a script, you want to write what is seen and heard. Her reaction to the shock by distracting herself is not unusual. It's interesting and deserves fuller development.
"Richard takes it, walks out of the room. Joan’s odd behavior had to be brought up eventually, right?"
What 'odd behavior' are you talking about? Doing the dishes? You need to show her doing something odd. And again, you're commenting in the script rather than describing.
The ending is very abrupt with no set up. Personally, I would have had an instance where when the husband walks in, the wife looks guilty and closes her laptop. I would probably place "clues" that suggested she was cheating on him. I would make the dialogue with his father more substantive. You have different directions you can pursue but if she's behaving oddly, the script needs to describe that.
As it is, this is more of a sketch of your idea rather than a workable script. Yes, you could shoot it, but it would be very unsatisfying largely because you've done very little character development, the dialogue is largely irrelevant, there is no set up so the ending feels forced and contrived. It should also be a bit more detailed in the setup of Adrian's disappearance. It could work well as a short with some revisions. Good luck.