IDK but if you've seen Abducted In Plain Sight, the parents allowed their daughter to go on overnight trips with the male, predator neighbor by themselves. they didn't call police either when he didn't immediately return her. They were clearly severely naive and i don't think it is far fetched to believe Steven's parents were naive in believing she'd be back eventually. Didn't a friend say that running away sounds like something she'd do?
It seemed like the mom was emotionally exhausted at being accused by her son. I saw her lashing out as being frustrated at the accusations. As if they wanted her to give them the complete story when she had none.
As far as the letter, didn't the handwriting analysis show it was likely Tony who wrote it?