So do you believe that ethics are just an invented rule system that could have a different form and still be as ethical?

What do you mean "as ethical"? By what meta-ethical rule?

if your reply is, "by the objective meta-ethics which I postulate that all sentient beings can derive" - if everyone can derive it equally, doesn't that imply everyone ought to be equally ethical? If you admit someone or some society are un-ethical (as you asked of Jack), does that mean they somehow failed to derive the meta-ethics? That the ethics they adopted is internally inconsistent somehow?