Ilúvatar and Melkor

[Kuruharan]

But would overthrowing Eru/Iluvatar be considered obedience or disobedience?

Most people would probably consider that a pretty extreme case of disobedience.

While Melkor could not overthrow Iluvatar he did hid the truth of Eru from those of the Children that he had power over, primarily Men, but possibly some Dark Elves too, and he filled their hearts with fear and darkness.

I doubt that Eru desired that the knowledge of himself be hidden from the Children, which is what Melkor did.

Think about it, why even bother to make the Children if none of them are ever even going to know about you?

However, overthrowing Eru was utterly impossible so it was probably just assumed, if never explicitly stated, that divine coups were off limits. Why bother with something you can't accomplish?