The subjective human ethics overlapping with the objective imperatives are actual imperatives; the rest are just preferences.

This redefinition of the word "imperative" goes counter to the existing meaning of the word (which would include all 'preferences'), so it's confusing. I suggest you come up with a new term or word-combination.

In the case of objective ethics, there is no contradiction if humans disagree about what is ethical; humans do not define what is objectively ethical.

You defined objective ethics as something every rational thinking being could derive. Shouldn't it also have some meaning? Some reason why they would in fact be interested in deriving it?

If this objective ethics can be derived by everyone, but happens to run counter to almost everyone's subjective ethics, why is it even interesting? Why would we even be talking about it unless we either expected to encounter aliens with subjective ethics similar to it; or we were considering adopting it as our own subjective ethics?

However, in defense of human ethics for either system, perhaps it is the case that human ethics are actually consistent, in a way that matters, but the terminal values are so higher order we don't easily find them. All the different moral behaviors we see are different manifestations of common values.

That definitely requires proof. Have you got even a reason for speculating about it, any evidence for it?