Possible reasoning behind "didn't want you to find out this way": He really didn't know what MIL was up to and meant he didn't want OP to find out about (supposedly) not being invited to christening. He was trying to spare OP's feelings for not being invited.
Doesn't excuse the deceit, but it sounds like OP's DH prefers to be clueless and stick his head in sand. He wouldn't have called his cousin, b/c he wouldn't want to get involved. He doesn't like conflict, especially when it's with his domineering mother. It was easier to go along with MIL and not tell OP b/c then her feelings wouldn't get hurt. (Or so he may have thought.)
Obviously H is not very switched on, as how could OP not find out about christening? That is the mind boggling thing.
THUM you have handled it all brilliantly so far. I hope that your DH decides to figure out what is important and apologises for lying and deceiving you. It is appalling, childish behaviour on his part and he needs to step up and stick up for you and YOUR family.
As far as MIL is concerned, it is so hard to take the high road, but I think you are right to. People like your MIL feed off of perceived weakness and if you go in guns blazing/crying/shouting/threatening, it may backfire. MIL can blame you for being horrible to her and alienate you from DH's family even more. It's all part of her plan, I'm afraid. People like this are very calculating as you've learned from her text.
It is a shame you feel you can't go to the christening (I would be thinking, "she's won, she's won! Can't let her win!" and would go anyway!) but do what you feel is best.