Just wanted to second everything Pommy said. You have no idea what your MIL really said and also what she's heard from him. You also know in your bones that what he's saying is BS-- the brokenness inside him is driving him to say terrible things in order to convince himself he's doing the right thing. The fact that he's continuing to be so unhappy and poisonous just shows what a bad place he's in.
My guess... though I know we're not supposed to be mindreading and above it all... is that by refusing to be his BFF and celebrate your BD with him, etc., he's seeing his fantasy D situation start to disintegrate. Of course that is awful for him, and it is all your fault, since if you would only have gone along with what he wanted and been a happy and supportive ex-W who is so glad to be BFFs with her ex and so glad he's found true love, he would have everything he wanted. Now he doesn't get that anymore and of course is blaming you.
I'm going to guess you'll be the recipient of more bile for awhile if you don't go NC. He has no-one else to lash out at but you, unless he's going to start looking at himself. That can't happen if you're still around to feed his narrative that you're the reason for all his problems. So I say stick with the NC as much as possible and just do your very best not to get drawn into his cr@p.
Hopefully, the NC and moving communication to email is letting you get some light into the situation and keeping you from getting sucked into his mess.
HUGS.
Me (46) H (42) M:14 T:18, D9 & D11 4/19 - 12/19: series of escalating BDs 9/20 - present: R and piecing
