So the question is less "will Trump give the order to do absolutely wildly evil things to Iran?" & more "what orders are going to be followed?"

And no, military commanders declining to obey clearly unlawful orders would not constitute a coup. That would still be the US military functioning within its designated sphere.

I'm not betting on the morality of the leaders of the US Armed Forces, but they have plenty of reasons to not want to carry out Trump's nonsensical revenge. So... we'll see.