I think the emphases and beats may be slightly different from performance to performance, but when I saw the show in Broadway previews, after Zoe said that Connor was a bad person, her parents started rapidly escalating on each other, essentially blaming the other for what Connor was like. The temperature rose very rapidly, and Evan jumped in with something like, "I remember lots of nice things about Connor" to prevent a full-blown war. Platt portrayed it as he says it very impulsively to de-escalate the situation, and I thought there was a little bit of an undercurrent that maybe this wasn't the first time he felt responsible for making the adults stop fighting. Right after he said it, in the moment when they stopped fighting and looked at him, Platt had this look on his face of "Uh oh, what did I do" as he realizes that now he has jumped into it with both feet and has to follow through.
I think Platt did a great job of showing how Evan got himself into a mess with good intentions, both in this scene and in the scene in the principal's office where he tries to explain that Connor wasn't the intended recipient of the letter but stops explaining when he sees how much the Murphys want the letter to be from Connor. I think the Murphy parents are pretty devastated in the principal's office. The rest of the cast also did a great job in those scenes. Later, Evan is more calculated about the lying, but in the beginning, he means well.