But then morality does not have as its subject matter....

I think you can keep that definition: define morality and morality-human. However, at least in the metaethics sequence, it would have done a lot of good to distinguish between morality-Joe and morality-Jane even if you were eventually going to argue that the two were equivalent. Once you're finished arguing that point, however, go on using the term "morality" the way you want to.

I only say this because of my own experience. I didn't really understand the metaethics sequence when I first read it. I was also struggling with Hume at the time, and it was actually that struggle that led me to make the connection between what an agent "should" do and decision theory. Only later I realized that was exactly what you were doing, and I chalk part of it up to confusing terminology. If you dig through some of the original posts, I was (one of many?) confusing your arguments for classical utilitarianism.

On the other hand, I may not be representative. I'm used to thinking of agent's utility functions through economics, so the leap to should-X/morality-X connected to X's utility function was a small one, relatively speaking.