Hugs, Lindsey. I'm so sorry that you're going through this. I hate, hate, hate when I get correspondence from XAH and his GF. Yeah, he finds a way to get around the civil No Contact order, too. But the civil NCO is different from a protective order and I'm told it takes a lot more to 'infractions' for there to be consequences. I've got a lovely list of their manipulative cr-p going. Thing is, it's fairly all subjective with the NCO and what the judge might see as abusive communication; we're allowed to communicate about DS. XAH manages to work DS's name into the correspondence so technically.....
It still throws my day/evening for a loop when I hear from them. I'm hoping for a day when it doesn't; for a time when I can see their names pop up on text or e-mail and go "Pfft. No big deal."
I completely understand not wanting to take action so that you can have a good day with the kiddos. I think it's a fairly natural reaction.
Personally: With the OFP, I'd report the contact. I'd journal everything, including the contact from your MIL on his behalf and note that she said she was calling because he asked her to. I think the next time she calls to pass along his manipulation, I'd inform her that the OFP stipulates that there is to be no 3rd party relays; if she presses on, I wouldn't be afraid to tell her "since you continue to relay information from him, this conversation is done" and hang up. Document, document, document if she continues to try communicate for him.
Hang in there. Wishing you peace and continued strength.