First of all I want to welcome you to the forums and thank you for your courage in sharing your predicament– this is absolutely the right place for this!
I'm sorry to hear about all this, it sounds very stressful and worrying. Here are my two cents:
Emotional blunting/detachment, as you correctly note, is a real phenomenon that is quite common in depression and other mental health conditions. It is also true that occasionally in romantic relationships feelings can suddenly disappear, but my hypothesis is that something different is happening here.
I think your friend and partner is reaching for language that she may not have and coming up short, and therefore deferring to the rhetoric of mental health and cognition. It sounds like the negative reaction she received from her family has made her doubt her decision to escalate this longstanding friendship to a romantic relationship, which of course is big leap on both your parts.
What you need to figure out is whether this is a case of cold feet that can be overcome, or if this is her way of saying that she is having serious second thoughts about this and would like to pump the brakes to preserve the underlying friendship, which no doubt is incredibly valuable to both of you. Either way, it is not fair to you to leave you guessing about her mental state, worrying what it is you've done wrong, and walking on eggshells as a result. That will lead to resentment and confusion, and again threaten this strong foundation of friendship.
I think the best thing for this is a kind but honest discussion where you ask here where she stands, and do your best to push for a better explanation than emotional blunting.
I hope you'll let us know how you go, and you're always welcome to share any developments or questions you might have.