Duties concern things that are voluntary. I do think that if you have a moral duty to bring me back the book you borrowed, that implies, roughly, that your doing so depends on your wanting to do so: if you want to bring me the book, you will. This is not the case if you are stuck at some airport due to a snowstorm, far away from me. This, however, is not the same as "ought" implying a metaphysical "can".

I don't think we are all irrational every time we fail to see through an argument in a book, but suppose it's true about you. You are still more rational than you think you are. You are irrational in a minor way - believing a misguided theory of the nature of rationality - but rational in a major way - you respond well to probabilistic evidence as you go through the day.

If I make a stupid decision but don't execute it because I'm, say, lazy, then I'm lucky, not rational. However, at other times a person acts for good reasons just as she does what she thinks she shouldn't do, not knowing that they are good reasons. Just like sometimes we are a lot less rational than we think we are, it is also true that sometimes we are a lot more rational than we think we are.

Suppose a student of mine writes in her exam that "morality is completely relative to culture, so nothing is absolutely right or wrong. Because of that, it is absolutely wrong to be culturally intolerant". This student, if she believes what she writes, believes a contradiction. She ought not to believe the contradiction - it's a basic epistemic norm. This is true even if she can't avoid believing it - no amount of studying will show her the light.

People of relatively low intelligence can be morally wonderful if they desire the right and the good (not necessarily under the description "right" or "good"). Their low intelligence sometimes results in their accidentally doing something wrong, but doing something wrong out of low intelligence alone is like stepping on a person's foot because you are (literally) blind or missing a cry for help because you are (literally) deaf. We do not judge the blind or deaf person as morally bad.

Incompatibilists will tell you that a work of art has no meaning unless the artists could have chosen to create a different one, but actual artists often say things like "the book chose me" - that is, the work had to be. Some philosophers would call it "volitional necessity", and a similar case that's discussed is the case of Luther saying "here I stand, I can do no other".

Virtue is about wanting right and good things, not about being particularly good at thinking.