I've been following this thread since last night and want to add my two pence worth.
Is it possible that MIL decided not to invite you along because of space in the car? With MIL, SFIL DH and DD in her car seat there wouldn't be room for an adult in the majority of cars so MIL might have decided it would have been easier to drop you. I know that's not nice but if she didn't expect you to find out then she's probably justified her actions to herself.
For what it's worth, I don't think she meant to send you that text, even if she truly hates you she would know that it was going to cause a row and she would come out of it as the villain as she orchestrated it. I think she just had your name in her head when she was typing the text and selected your name by mistake, I've also done this before but luckily I wasn't slagging anyone off
I also don't think your husband has acted particularly deviously, he's been a bit of a twat but aren't all DH's like that at times? If MIL told him you weren't invited he was probably went along with her plans to spare your feelings.
He sounds a lot like my DP, in that he hates confrontation. He's probably not the type to confront your cousin about not inviting you, he showed this by hiding in the bedroom rather than speak to you about it when it all kicked off. He'll also hate the thought of confronting your MIL about her behaviour for the same reason.
I agree that he does need to speak to his mother but if it was me I'd bite your tongue and act very graciously about it. If you go around shouting the odds at her you'll look like a fishwife and give his family a reason to hate dislike you. Use this to your advantage and get your DH to gently lay down some new ground rules, play the upset wife rather than the angry wife then he can explain to his mother that he doesn't ever want to upset you like this again.
I hope you get this resolved in your favour, it's definitely not worth ending your marriage over.