I do see what what she wants (which is English lit at Cambridge, though she believes herself to be incapable) is quite clearly not what she needs - despite her stellar academic record and crazy ability
As others have said, university (whichever she chooses) is not the issue here. It's her fragile mental health which leads her to believe herself "incapable" despite all the solid evidence to the contrary.
Frankly, she is not a "teachable" student at the moment in university terms. University students need to get rid of the false modesty that our school system seems to encourage, especially for girls, and they need to start developing a more mature ability to assess themselves professionally. It's one of the toughest things they have to learn - that by the time they graduate, they should not be seeking their tutors' approval, but they should be self-reflective and realistically self-assessing.
Someone who catastrophises - "I'm no good" in order to seek reassurance (which I think is part of what your DD does) will not be in a position to learn independently in the way that a talented excellent university student needs to learn.
And I'm afraid it sounds as though you unwittingly encourage this catastrophising by your constant closeness & reassurance (it may of course be quite different IRL, not written down). She really really needs good therapy. I understand the sense of helplessness & stuckness in an abusive marriage (one only has to read the Relationships Board here), but you also have to recognise the harm it did to your children. You may have been helpless and abused, but you were the grownup, and your children were only children. I find it extraordinary to read how detached you are from the idea of deep lasting harm that you staying in your abusive marriage did to your children. But I guess that's a coping mechanism, and the real blame lies with the bastard who was their father.
However, you can't assume that because you coped (only just, I'd say) the children will also cope.
For me, this is part of the other huge issue - yours & her co-dependence. I know you resist this, and I understand that it could be the effect of writing things down for an anonymous internet message board. It may be different IRL. Except that writing anonymously can be a kind of revelation of truth for yourself.
The other thing to say about this thread is that sometimes, when you ask people's advice, and hear things you don't want to hear, or have a gut feeling about, it's worth listening to your own instinctive response.
Try to work out what your emotional, instinctive response is. I'd say that THAT is what the real "problem" is.
What is it that you want to protest most strongly against from what people have said to you on this thread? (Please don't answer this question here, take it away & think about it). I'd bet that that is what needs thought & work & therapy. The depth of your denial is indication of the extent of the issue, I'd say ...