Whilst I agree that THUM's DH burying head in sand behaviour is typical of some men it doesn't mean that it is right. It shows disrespect and complete immaturity. Yes he has been backed into a corner but whose fault is that then? The MIL does sound poisonous but the DH went along with it. He knew it was wrong as he didn't want THUM 'to find out like this'.
Find out what exactly? That she wasn't invited to a family event? - No - she was but for some reason he chose not to pass on the invite. Why? Because MIL suddenly wanted him and the DD to herself? It's all a bit odd - surely the MIL would expect other family members to wonder where THUM was on Sunday? And why send the text to THUM? Very strange behaviour.
The MIL appears to have form for being controlling and the DH has grown up with this and it is normal to him. Up until now (we think) THUM hasn't been on the receiving end.
THUM - I hope you got some sleep. I feel really sorry for you having the rug pulled out from under your feet. You and your DH really need to sit down and talk about this. I do agree that you will get further with him if you stay calm. Difficult as it may be.
I would say to him that you were extremely hurt and angry to find out that he and MIL were doing this to you. That you are wondering why. Two scenarios jump to mind - he wanted you out of the way as he has someone else who will be there (that in my view IS an option I'm afraid - it could be a family member or friend unfortunately). Other scenario is that MIL didn;t want you there for some reason and he went along with it. He went along with the 2 possibilities - either you were not invited by cousin or MIL didn't want you there.
He knows full well that it is wrong. That is why he is hiding in bed.
You need to ask him who his priorities are - his mother or his wife and child. You need to spell out to him (still calmly if you can manage it!) that his mother's behaviour is not normal. I would also ask him if anything similar has happened before.
I would tell him that you love him and that he is a good DH and a good Dad but that you need reassurance that YOUR family unit comes first. And I would be extremely and completely clear that if anything like this EVER happens again that you are finished.
I would then send MIL a text along the lines of 'I am not sure what games you are playing but I am shocked and hurt and angry that you would do such a thing. We will not be seeing you for a while'. See what the response to that is.
REgarding whether you go on Sunday. Up to you. Personally - I wouldn't. I would call the cousin and explain that you really don't wnat to be in the same room as MIL and you don't want an atmosphere at her DC's christening. I would then buy a card and present and drop them off at cousins.
And I would tell any family member who asks exactly why you were not at christening!