Okay, I just think you seem to have some pretty radically different intuitions about what counts for someone's well-being.
One other thing: you seem to be assuming that the only reasons someone can have to act are either
I think this isn't true, and it's especially not true if you're defining well-being as you are. So you present the options for P as
but these aren't exhaustive: P could just want the cube to exist, not to produce mental states in themself or for a moral reason. If you're now claiming that actually noone desires anything other than that they come to have certain mental states, that's even more controversial, and I would say even more obviously false ;)
I said that there could be other reasons for P to want the cube to exist. If someone has a desire that fulfilling will not be good for them in any way, or good for any other sentient being, that's fine but I do not think that a desire of this type is morally relevant in any way. Further if someone claimed to have such a desire, knowing that fulfilling it served no purpose other than simply fulfilling it, I would believe them to be confused about what desire is. Surely the desire would have to be at least causing them discomfort, or at least some sort of an...
I figure morality as a topic is popular enough and important enough and related-to-rationality enough to deserve its own thread.
Questions, comments, rants, links, whatever are all welcome. If you're like me you've probably been aching to share your ten paragraph take on meta-ethics or whatever for about three uncountable eons now. Here's your chance.
I recommend reading Wikipedia's article on meta-ethics before jumping into the fray, if only to get familiar with the standard terminology. The standard terminology is often abused. This makes some people sad. Please don't make those people sad.