TL;DR - If Trump wants the military to get involved in the situation, he would need to have (or he would appear to have) the support from the majority of the public.

I had a couple of all-out discussions on the similar topics with people in my work sphere.

POTUS losing his platform (by or because of domestic threats) would cause a military takeover?

No. Even if there's a law which can be interpreted as "unlawfully imprisoning the high-level American officials may call for a military intervention as a conflict resolution," the word is it won't happen this time for Trump.

I myself didn't know this: Nixon didn't want to give up and was intended to fight till the end.

Then, the top military brass (George S. Brown, Creighton W. Abrams, James L. Holloway, & David C. Jones to be exact) asserted to Nixon that they won't be involved. Their reasoning (or excuse, depending on how you want to see it) was said to be that Nixon was elected unlawfully. After he realized the military won't be sided with him, Nixon gave up.

Yea, just like what you think as reading this, my response was; "Exactly! Biden was elected unlawfully! No?"

No, that's not the point. The decision point is "It's not up to Trump. It depends on how the military see the current situation. Is it going to be necessary? And more importantly, until the military believe they got the support from the public, they won't budge an inch.

Whether or not manufactured, do we as the US public believe the Trump team (appears to) have the public's support now?

I think this is why we are witnessing the unification by all the establishment factions and their singing a song that Trump is a domestic terrorist. spits on the ground