I think Elayne has the correct point here.
Mutual benefit is not "hopelessly vague". In fact, it made it to the last ten Principal Doctrines
I do not see "mutual benefit" as written into the last ten principal doctrines. The last ten essentially state that there is no such thing as absolute justice, which is Elayne's point. Now if the parties involved in a relationship agree to certain terms (not to harm or be harmed) then that is what we call "justice," but if the two parties end their agreement, for whatever reason, then there is no more justice. That's really all the last ten are saying, they are NOT saying that a particular set of facts constituting justice "is always good" or "injustice is always bad" any more than any other set of facts are laid out to be good or bad in the context of any other virtue.
The entire point of the virtue analysis is that there IS NO absolute virtue.
But Hiram you take from that starting point that you should endorse particular policy prescriptions that apply to everyone as something that would be endorsed in the name of Epicurus???
I do not follow that analysis at all!