Per usual when people besides OP respond in this subreddit, you have missed my point entirely. I've had this experience a number of times before, so I've learned the best way for me to move forward.
If it keeps happening to you, perhaps you need to consider that you aren't always correct and could stand to be less offended by open conversation. Certainly, I would not continue to respond like this at the very least; whether you agree with /u/riot_act_ready or not, they did at least take the time to formulate a response to the thread and not the act of disagreeing itself.
And for what it's worth, I've never seen this forum operate with the kind of structure you describe - singular comments with no discussion or disagreement. I don't think the rules mean no one can disagree, they mean don't be an asshole and I don't see anything wrong with the other poster's reply.
Having said that, /u/riot_act_ready I agree. You simply cannot compare how modern families and relationships work to tribes. It just isn't reflected in how families form, break up, or even live. It is common, for example, for kids to go away for college and just never return home. When was the last time you heard of a cohesive "tribe" scattering across continents? No, the need for tribes is gone and this is reflected in society. There is nothing wrong with choosing to bond together, up until you believe it means you need to swallow up the toxic behavior of another person. You can make that choice for yourself but I am very strongly against judging someone else for making a different call.
OP is, of course, welcome to seek therapy and even ask her mother to go with her if this is the sort of relationship they have. But it doesn't sound like it is. As a reminder, this isn't just about issues they may have as mother and daughter, this is about the way her mother behaves as a person and you'd have to be pretty damn narcissistic to think you can force an adult into therapy. As a recap:
She even pulled me aside to talk to me about life insurance and wills, saying that "whoever I marry", she wants to ensure a good life for me and her future grandchildren, and not any step-grandkids. She outright warned against leaving money to Frank or sharing my resources with him, because "what if you die and he remarries and has kids with the new wife? She'll spend all the money and he won't care about your kids". This isn't the first time she has basically coached me to assume everyone, including my future husband, is out to get me, and that I can't trust anyone except (blood) family.
These are her mother's issues, likely deep-seated. I'm not sure what a person could expect OP to do as she begins planning her own wedding, potentially going to pre-marital counseling during that time, and starting med school once she's accepted.