Well at least we got past the domestic territory thing. It isn't the "military's point of view"; as you pointed out the military is under civilian control and all the restrictions and training on those restrictions comes from the civil law that governs the military's actions. We didn't do all that noise with the Coast Guard detachments because the Navy and Coast Guard got together and decided it was a great idea!Certainly what you say is true from the military's point of view. I don't question that one bit. But in the end it isn't the military who decides these matters. (well maybe in the very end it is!)
Remember Eisenhower (who likely had similar briefing on Posse as you) sent Federal troops into Little Rock! And Arkansas is not even a territory, it's a State!
Yes, once in our history a President did what would probably be determined illegal if it had been taken to the supreme court in an extraordinary circumstance. One could do so again, but unlikely that PR rises to the level of extraordinary circumstance for a President to risk falling on their sword over, especially when Marines are such an inappropriate choice of tool to use in that circumstance.