If this was my MIL, this is how it would go down. Once I made it clear to her how I felt about the situation & she did not relent, it is then my husband's job to deal with his fracking mother. Whether she likes it or not is completely irrelevant & nothing she does or says holds any weight.
That's me. For you, I say - this is YOUR time & YOUR child. She can trust in her son to have made good decisions about the woman he married & how he & said woman will raise the next generation or she can spike her blood pressure either way it won't make a lick of difference. She's shouting at the rain. If she's unstable, then I say don't tell her where you're delivering, ignore the questions about plane tickets & don't inform her or anyone who would contact her about when you go into labor. But most importantly, make sure your OH is on board with you... nothing worse than a momma's boy caving under her & behind your back.
My MIL drives me a little bonkers too - but after 3 years of being married to her son I've got her trained. Good luck!