Congratulations on the pregnancy! Yes it's disappointing that the defence hasn't happened yet but it sounds like at least concrete steps are being taken to make it happen - it's important to keep them in track. For the purpose of clarity I do think your Mil has a pretty fair point about the degree, but it sounds like the family problems go pretty deep and this is just one manifestation of a wider issue
Apologies for the long response but we experienced somewhat similar issues with a parent when my daughter was born. At the worst I thought we may need to cut off contact, but we were able to turn things around and in doing so transformed the relationship with the parent. Seeing how much they love and care for my daughter has also helped us appreciate them more than we ever did. But you do need to address this situation and do it now, not in 6 weeks time when you're both sleep deprived, exhausted, hormonal and Mil is making things harder for you not easier.
Having recently lived through the early parenthood days they are wonderful and rewarding but will put a lot of strain on you, your wife and your relationship. Having family near by can be a huge blessing and is not to be underestimated. However if things have been difficult to this point don't think a kid will make them easier. There are lots of touch points that can cause disruption - but on the plus side it is a great opportunity to reframe your relationships with MIL. She's evidently big on parental authority - good, use that. You're the parents now. You can explain that you want her to be a big part of her grandchild's life, but that she needs to respect your roles as the parents and not undermine you both.
Basically what you need to do is to set healthy boundaries - I'd urge you to read up on this or even if you can to attend a session or two with a counsellor who can help you do this, along with your wife as it's something you need to do together. Up thread you said that your Mil doesn't like to be educated - the beauty of setting boundaries is that you don't need to educate her. You don't need her permission or her buy in to do it. Setting boundaries is something you and your wife do together, for yourselves. You don't need mil to change her behavior or understand how she makes you feel. It isn't even about the Mil, its just about the two of you deciding what behaviors you will and won't tolerate.
So for example, you don't ask her not to nag your wife, or try and explain how much it hurts your wife when she does. You don't try and change her, because you can't. Instead you decide a boundary and what your response is, and explain that to her dispassionately. For example tell her firmly but non emotionally that the defence is being scheduled and you'll tell her when you have news. In the meantime its not a topic you're prepared to discuss. Explain the consequences of she does raise it, for example youll politely remind her that it's off the table but if she persists you will have to disengage from the conversation by leaving the house of hanging up the phone. If this happens you follow through, not just in an emotional way. It's just the unavoidable logical consequence of your boundary being over stepped.
It's easy in theory but does take commitment - from both you and your wife to implement. It can be scary at first and initially she is likely to escalate her negative behavior in the hope of eliciting a response. This is where it's really important that you stick to your guns, don't be swayed and don't get drawn into am argument. Firm and dispassionate. Most likely she will pretty quickly learn to play by the new rules, her desire to spend time with her grandchild will help her find a way. This can get messy in the short term which is why I suggest counselling or at the very least reading proper books written by people with qualifications rather than random strangers on the Internet. But it can be life changing.