Good question, DC, and I agree with all the views expressed. The combination of all those factors is what makes the scene so memorable.
Another thing: While Sonny (earlier) had said that the Don "will have my head" if Michael got directly involved, Tom may have felt that the Don would hold him, not Sonny, personally responsible if anything bad happened to Michael. As we saw in the flashback scene at the end of II, Tom told Michael "your father and I have talked, many times, about your future." So Tom may have been thinking that the Don might have expected hothead Sonny to lose his cool and get Michael involved. But not Tom... And, as we saw later, when Vito comes home from the hospital and asks about Michael, it's Tom who tells him that Michael shot Mac and Sol. Vito then musters as much anger as he can at that point and waves Tom away.