I'm not sure why it's necessary to use 'should' to mean morally_should, it could just be used to mean decision-theoretic_should. E.g. if you're asked what a chess playing computer program should do to win a particular game, you could give a list of moves it should make. And when a human asks what they should do related to a moral question, you can first use the human_right function to determine what is the desired state of the world that they want to achieve, and then ask what you should do (as in decision-theoretic_should, or as in what moves/steps you need to execute, in analogy to the chess program) to create this state. Thus morality is contained within the human_right function and there's no confusion over the meaning of 'should'.
As long as you can keep the terms straight, sure. EY's argument was that using "should" in that sense makes it easier to make mistakes related to relativism.
In You Provably Can't Trust Yourself, Eliezer tried to figured out why his audience didn't understand his meta-ethics sequence even after they had followed him through philosophy of language and quantum physics. Meta-ethics is my specialty, and I can't figure out what Eliezer's meta-ethical position is. And at least at this point, professionals like Robin Hanson and Toby Ord couldn't figure it out, either.
Part of the problem is that because Eliezer has gotten little value from professional philosophy, he writes about morality in a highly idiosyncratic way, using terms that would require reading hundreds of posts to understand. I might understand Eliezer's meta-ethics better if he would just cough up his positions on standard meta-ethical debates like cognitivism, motivation, the sources of normativity, moral epistemology, and so on. Nick Beckstead recently told me he thinks Eliezer's meta-ethical views are similar to those of Michael Smith, but I'm not seeing it.
If you think you can help me (and others) understand Eliezer's meta-ethical theory, please leave a comment!
Update: This comment by Richard Chappell made sense of Eliezer's meta-ethics for me.