The usage in Stuart's posts on here just meant a certain way of calculating expected utilities. Selfish agents only used their own future utility when calculating expected utility, unselfish agents mixed in other peoples' utilities. To make this a bit more robust to redefinition of what's in your utility function, we could say that a purely selfish agent's expected utility doesn't change if actions stay the same but other peoples' utilities change.
But this is all basically within option (2).
No one can mix another person's actual utility function into their own. You can mix in your estimate of it. You can mix in your estimate of what you think it should be. But the actual utility function of another person is in that other person, and not in you.
Human values seem to be at least partly selfish. While it would probably be a bad idea to build AIs that are selfish, ideas from AI design can perhaps shed some light on the nature of selfishness, which we need to understand if we are to understand human values. (How does selfishness work in a decision theoretic sense? Do humans actually have selfish values?) Current theory suggest 3 possible ways to design a selfish agent:
Note that 1 and 3 are not reflectively consistent (they both refuse to pay the Counterfactual Mugger), and 2 is not applicable to humans (since we are not born with detailed descriptions of ourselves embedded in our brains). Still, it seems plausible that humans do have selfish values, either because we are type 1 or type 3 agents, or because we were type 1 or type 3 agents at some time in the past, but have since self-modified into type 2 agents.
But things aren't quite that simple. According to our current theories, an AI would judge its decision theory using that decision theory itself, and self-modify if it was found wanting under its own judgement. But humans do not actually work that way. Instead, we judge ourselves using something mysterious called "normativity" or "philosophy". For example, a type 3 AI would just decide that its current values can be maximized by changing into a type 2 agent with a static copy of those values, but a human could perhaps think that changing values in response to observations is a mistake, and they ought to fix that mistake by rewinding their values back to before they were changed. Note that if you rewind your values all the way back to before you made the first observation, you're no longer selfish.
So, should we freeze our selfish values, or rewind our values, or maybe even keep our "irrational" decision theory (which could perhaps be justified by saying that we intrinsically value having a decision theory that isn't too alien)? I don't know what conclusions to draw from this line of thought, except that on close inspection, selfishness may offer just as many difficult philosophical problems as altruism.