First, your deep question deserves a better answer than I can give. I have asked a couple of people who have thought a lot more about our moral responsibilities to weigh in, and hopefully they will help out. Please, do check back. At the same time, I thought I would put down some thoughts on this, and keep it on the blog so that the thread remains.
I appreciate what you are trying to do here. I wrote a blog primarily from a positivist account of the world, because that was the premise of the original article that I was critiquing. You are arguing that such a positivist account is inadequate for the problem at hand, since deep normative issues are at stake. One simple response, that is surely unsatisfactory, is that economists' are less comfortable with normative arguments, although they are clearly required for important societal decisions. Kaushik Basu's paper on "Gender and Say" is precisely one example of how normative accounts are required for analyzing the validity of certain voluntary contracts.
But we can go further. I am not sure whether the dichotomy you have created between welfare/preferences and moral rights/responsibilities is necessary in this context. It seems to me that the problem remains when taken back one step, by asking: "Whose moral account of the world are we interested in?" After all, we know that much harm has been perpetrated by those who believed strongly in the universality of their moral universe, and were deeply committed to helping the poor, and even further, all of humanity itself. One has to but read Matthew Conolly's chilling account of the family planning movement (Fatal Misconception) to realize that those who were at the helm of the movement indeed believed that their moral accounting was beneficial for the lives of the poor around the world. By posing your question in this way, you perhaps lead us into a moral absolutist position that neither of us holds.
But then, are their limits to moral relativism that a rejection of the absolutist position would demand? Here, I really liked Steven Luke's short book on the dilemma of the relativist and his answer is this: He would like us to look at the problem from the viewpoint of those who are the ultimate receivers (or, as often happens, those who suffer from a norm/policy) and elicit their preferences in assessing a moral relativist position. For instance, in the case of female genital cutting, he asks whether those who are subject to the procedure would agree to thus be subject. And if not, he would ask that our moral relativism not take us so far as to justify such a procedure on the basis of cultural norms.
Take it back to economics, and we are firmly in the realm of representation problems that have raised their head before. Who represents the poor that we want to help? In the case of the family, and giving money to a parent who spends it on alcohol rather than schooling, as economists we worry when this represents a breakdown of the family, whereby the parents no longer represent the interest of the child. In a positivist account, this would be a market failure that has arisen previously in the comments thread. The beauty of market failures was precisely that we never needed to measure welfare in an objective manner.
What I would then claim is that the positivist account, is thus far, at least somewhat more powerful than we may give it credit for. I would still maintain that the original premise of my blog is correct: It is far more interesting--and important--to frame the question in terms of market failures, or, indeed as you have done, in a moral rendering of the universe than in the simplistic fashion it was originally presented.
This can then lead us into the deep issues of representation, democracy and the circumstances under which that representation breaks down, which should surely be the focus of our inquiry.