Update: January 13, 2019
Fareed Zakaria covered this on GPS today with the Atlantic writer and one law professor. The general impression is that Trump's authority to reallocate monies for the Wall as less clear legally than some of the other powers, such as severely restricting the Internet. The biggest danger is that Trump finds another, more credible emergency to attach this border problem to, and I can certainly think of some of these myself. This is "Milo"-dangerous stuff indeed, and Congress needs more control of the president than it has. And the GOP Senate needs to step up and become adults in the room, really fast.
Cato Institute published a discussion by Gene Healey of the Atlantic article. I didn't pick up her speculation on preparation for war with Iran as the pretext. North Korea is much more dangerous (and we barely missed war late last winter, probably). A competent authoritarian might have turned off a lot of the Internet by now, as Cato notes. You listen to vlog videos by "Economic Invincibility" and you realize he could get a Wall built if he were in office with very little trouble.