I found episode 5 kind of fascinating and I'm still mulling it over in my head. The point was made that Katherine fired a former employee specifically for lying. As the episode progresses she learns that David is a big liar and yet she slowly seems to accept and even defend that lying maybe isn't that big of a deal. He's an interesting guy - it's just a quirk of his... And Alex, who clearly dislikes David in the beginning, gradually starts to accept the same behavior that he found so gross earlier in the episode. He waves off the possibility of David giving alcohol (or was it just beer??) to a minor. He even kind of laughed about falsely accusing someone of doing something awful in order to get out of trouble wasn't so bad. He would have done the same when he was young!
Then there was the flashback scene of Alex being chewed out by his father after he asked a friend if they were a boy or a girl. Grown-up Alex understood that his dad's harsh reaction came from the desire to make him a better person. So, when Alex sat his boys down, his speech went in a totally different direction than I expected. I thought he was going to cut through the bullshit and offer them a safe environment to confide in him regardless of what (if anything) had happened, but instead he completely shut it down. And there were definite red flags, like the boys wanting to skip their lesson, or the middle-aged boy acting so angry and defensive when asked about it, that should have been investigated more by the parents.
At the end when Katherine slowly shuts the door on the piano lesson, I interpreted that as acceptance - on "closing the door" to the willingness to question or suspect any past or future wrongdoing.