Re: There's just one problem with this scenario...
nor can any POTUS simply declare a national emergency
No need. We're in a state of emergency already. We're in 30 of them. We've been in a state of emergency since 1979.
Frankly, your whole post smacks of "it can't happen here". While I'd certainly like to believe that's true - and I for one have viewed all the predictions of doom since 2016 with a jaundiced eye, even while acknowledging Trump's many vile deeds - I also remember how every failed state has had no shortage of people explaining why their country could never devolve into autocracy.
My suspicion is that Trump's handlers have enough control over him to keep him from doing anything that might upset the Wall Street applecart. The Mercers, for example, presumably enjoy the bull market and want at least the pretense of constitutional government to continue. And Trump's quite comfortable right now with the Senate and SCOTUS on his side. But I'm not ruling out an attempt to assert excessive power, if only because I don't think he's rational.