FrancisdeSales I am sorry to hear this.
It sounds very unfortunate for you and I hope someone with better advice will be along soon!
For me, here is what i would like to say to you.
No, you are not 'co-dependant', at least as far as I understand it.... "Codependent relationships are a type of dysfunctional helping relationship where one person supports or enables another person's addiction, poor mental health, immaturity, irresponsibility, or under-achievement."
But your husband has a big problem with his mum and he really needs to work this out, with your help, is he willing to work it out?
I would say about the car, can he/you afford to give/lend this car to your MIL? If yes, I would try and not focus too much on the specific car aspect. If not, then that is an issue and he needs to see his part in this is very unhelpful.
He has let you down about the anniversary, will he see that, will he feel guilty but be unable to make a change?
I think you need to reassess this calmly. He's been like this a long time, this may feel like a final straw or something major. In your shoes, I think, I would want to talk calmly to your friends and try and get this into perspective. Can you live with this, can you change him, can he change? Just work out calmly whatever is best for you and your children.
Good luck.