To those people who think the MILis sadly misunderstood and needs loving indulgence to make her feel secure - why don't you go and spend some time with the vindictive cow?
People like this may need help and lots of cuddles and reassurance etc, but they are malicious, self-obsessed, spiteful and cruel - they can't see past what they themselves want and will do their damnedest to destroy the relationships and confidence of those around them so that they can achieve and maintain control.
They refuse to acknowledge that they need help, and won't seek it. They don't like anyone to have autonomy and will do ANYTHING to prevent the people around them from living happy lives because they thrive on conflict. They will find a way to ruin EVERYTHING for others - trust me I've had a toxic family member like this, and there is no length they won't go to to blight lives.
OP has tried to be friendly - even affectionate (asking her to be her birth partner) - but MIL has thrown this back into her face. There are children involved in this situation, too - they are entitled not to have their early years poisoned by this venomous , and don't need to get a lesson in being a doormat from parents who kow-tow to a bully.
OP - your DP sounds a lovely man who has his priorities right. Marry him; cut his cowbag mother out of your lives; keep her away from your children (because she will corrupt them and do her best to turn them against you, and then she will destroy their lives, too); don't engage with her - if you meet be polite but as distant as you can be, and if she is rude/self-pitying/aggressive, then remove yourself and your family from the vicinity.
She won't die - sadly 
- and will move on to another victim - but it won't be you and your loved ones.
Don't let her ruin your relationship.
(And those people that say "you're long time dead" (paraphrasing) and you should all kiss and make up - there are some people so horrible that life is too short to spend time with them. OP's soon-to-be, we hope, MIL is one of them)