Face to face is the way forward I think! As they say only 90% is the words you use, it’s all tone, body language, facial expressions. She’ll see that you’re hurt, anxious etc and hopefully share more empathy towards you. I get that it’s difficult but I’d think try to do it asap, then hopefully last few days you can start to rebuild some type of relationship. It was definitely right of you to say no to all squeezing in the car, naughty of her to try force you to. Husband should’ve also said no to be fair. However I think coming from you makes more of a stronger point. Example; my partners Nan posted my LO in the bath on fb, could see her noonie (and we also just don’t post her online) he said ‘Nan you can see her privates can you take down.’ She apparently went ‘no you can’t it’s fine’ I didn’t even know if this then I see the photo, spoke to him he told me so I messaged her. She straight away deleted it. Stand your ground and share your feelings
To me, your husband is the one who should be leading the way with this conversation, especially where safety is concerned, its not up to you to write a letter!