I'd like to help you with your question.
The situation you described is not only frustrating, it is unhealthy. Yes...her father is manipulating her and she has become his "wife". I can't stress enough how extremely unhealthy this relationship is. Yes...she needs to get professional help so that she can unhook from him and live her own life. Until she does that, she is unlikely to have the emotional strength to leave the situation.
As you noted, she is defensive when people point out how manipulative her father is and she has ended relationships over it. That's a sign of how trapped she is and how unable or unwilling she is to change the situation.
I don't think you have alot of options here. I'm sorry.
In my experience, she is not going to be able to commit to a marriage or even a healthy relationship until she is free of her father. Even then, she will have problems committing because of how abusive and damaging this has been to her self-worth and self-image. Does this make sense? She needs help and she needs it sooner than later!
You have no ability to change the situation. The only person you can change here is you. You can't make her get help, you can't make her leave him...she is going to have to want this for herself.
So...what to do. You could tell her that you want her to go to therapy so that she can see how destructive her relationship with her father is and so she can gain the strength to change. It could be go to therapy or our relationship is over. I'm not sure if you want to go that far or if you are at that point. If she refuses therapy are you ready to walk away?
Otherwise, you could go to therapy on your own to gain the emotional strength to stand up to her and her father on this issue. I am not sure if you want to make that kind of investment. And...you still have no power to change her - but you have the power to make changes for yourself. Remember...you have been impacted by this manipulation too! From what you write, you have a very limited ability to be together and to build a life together.
If you do want to marry her and start a new life, that cannot happen without her committing to YOU. Right now her priority is her father and that's where her commitment is.
This sounds like a terrible situation.