Even comprehending the "open question" issue is beyond my present intellectual capabilities, so I'm not that interested in it. But the reason that I cannot accept an account of morality that is purely descriptive is that morality is prescriptive, not merely descriptive. It prescribes how we ought to act in the future, which an account of brute natural facts can never reach or address. There is no value neutral argument for an evaluative conclusion. You can never infer any ethical conclusion from any set of entirely non ethical premises. In order to get to an evaluative conclusion you have to presuppose it, but that's the very thing you're supposed to be explaining.
It is a category mistake for you to ask if the utilitarian principle I proposed being acted upon by rationality+culture+innate values is good in the context of this naturalistic account of morality...
If I were asking about the color 'red' it would indeed be a category mistake to evaluate it in terms of good or evil. But how is saying something is good or right or virtuous a category mistake when discussing morality? Your position is that good and right or virtuous can be thoroughly described in terms of nature features. Please explain how it is then that when speaking of morality we seem to be saying something different from or more than what we are referring to when we speak of a color. How is any brute natural fact connected to whether anyone ought to act or be a certain way?
I also observe that you smuggle in an evaluative premise with, "...+innate values", but values, for example, "good" and "evil" is what you're supposed to be explaining in the first place empirically, by natural facts. You know, chemistry and physics and the like.
Cordially,
It's too bad you can't expand your thinking, get outside that rotating frame of reference. The explanations are so mich simpler in an inertial one.