Collect the letters, save emails, texts, and let phone go to voicemail. Once she threatens YOU in anyway, YOU can file a restraining order against her. This will keep her away from you.
Until court, I would delete your facebook account. Not worth it.
A lot of ignoring but it is hard when you are in the middle of trial.
You can ask that pick ups be at school or daycare so she is not there. If your husband can do exchanges without you, do that ( maybe he can have someone else there). Protect yourself, tape all exchanges. You can ask to do exchanges in police parking lot too.
If his ex is like ours, she grills kids when they came home and finds at least one thing to complain about. Just try to ignore, take lot of pictures of normal stuff, baking cookies, playing board games, going to park. In court, bring pictures of his current room, enter them into evidence.
You are her target so take you away a little. You should not be answering her at all, talking on phone, sending letters back to her. Just laugh at her craziness and move on. If it gets ugly from her, take evidence to police station and ask for harassment charges against her.