I think you have managed to successfully demonstatre only the limitations of language, or, at least your use of it.
You would agree with me, I hope, that there is no single “right” way to play chess nor a single "correct way to respond to an essay question, yet, there are good chess players and people who flunk examinations.
Accordingly, morality is a “correct” way to behave. Thus, borrowing on the chess example, we elect to proscribe theft because it doesn’t work well to allow it, not because of the reputed etchings on a stone tablet.
But we say it doesn’t work well
because we already have an idea of what it means to work well. It is this we are challenging you on. We all know that allowing theft doesn’t “work well,” but this is because we share, more or less, the idea of what “works well” means. But we don’t accept that you have given either a valid reason to accept this idea of working well (that we do in fact accept) or any particular reason why someone is bound to do stuff that works well, even if he agrees with you on what that means.
It appears that you are introducing an absolute of sorts, but trying to avoid calling it that and saying that it doesn’t really come from anywhere important, but that it still for some reason is kind of sort of binding.
You mentioned chess. The goal of chess is to check-mate the other king. Now there are lots of ways to do this, some better than others and some just really, really bad, but these value comparisons all depend on the fact that goal is to check-mate the other guy’s king.
And why is the goal to check-mate the other guy’s king? Because it’s written on a stone tablet, so to speak - it’s in the rules. But we made the rules and only play by them because we agree to. So what would you say to someone who decided the
real goal of chess was to have all his pawns die horrible inventive deaths? He would be playing wrong according to the rules we like, but why does that actually matter at all, either to him or to us? To say that it matters simply because it goes against the rules are to elevate the rules, or at least the idea that we should follow the rules (or possibly the idea that we should follow some rules) to the level of an absolute turth, but where does it come from in a world with no objective absolute meaning?
Or in other words, we’re back to "why should I care what you consider to be ‘working well,’ and even if I agree with you on what that means, what is there about working well that compels one to try to achieve it?
That there are multiple ways that are acceptable is completely irrelevant by the way, there are also multiple ways I can throw a ball but no matter how I do, the ball will obey the absolute laws of physics.