I guess another way of asking the above, is that if the good is to be rational, according to Aristotle, along with a disinterested concern with the good itself, according to Plato, then why is there so much confusion about what a person should do?

There is confusion because ethics is not about convincing people what to do. It is about deciding what is the right thing to do. Plato believed that if a person knew the good, that person would invariably do good. There is no evidence that this is so.

Every person's fundamental option is between seeking the good and and being free from the constraint of good and evil. Aquinas speaks of it as a commitment to God. To be committed to God, one does not need to know that it is God that one is committed to. This distinction is made in the judgement story of Matthew 25, where many of those judged did not know who they "served." (I'm not making a faith claim, but citing a historical example of the distinction.)

A second problem is the contemporary belief in Hume's, or in G. E. Moore's, "Naturalistic Fallacy." Hume believed that one could not conclude what "ought" to be from what "is." Moore believed that "good" could not be defined in terms of what is. This effectively disconnects morality from cognition. Since we can only know what is, we can't know what is good.

A third problem is the prevalence of irrational ethical theories. I am thinking specifically of various forms of utilitarianism and consequentialism. Utilitarians insist on optimizing some utility function, assuming without warrant that it is meaningful to speak of optimizing something that cannot be measured, or in many cases, even rank-ordered. Consequentialists want to judge moral decisions on the basis of consequences that, in many cases, are unknowable at decision time.

Our concern for the good need not be "disinterested." For one who has chosen the fundamental option of seeking the good, seeking it is the essential to self-realization.