If it's a form of "regulation," then Trump wins.

If it isn't, he loses.

I think he wins. If a tariff isn't a "regulation," then what is it?

A tariff is almost universally understood to be a tax.

You've inverted the question from how it was considered at SCOTUS, though. The issue isn't whether a tariff can possibly be considered a regulation. It's whether the grant of the power to regulate includes the power to impose a tax. IOW, not whether a tax can be used as a tool to advance a regulatory goal, but whether the grant of the power to regulate something like trade would ordinarily be understood to include the power to tax things, rather than other actions that are more commonly understood to be regulation.

That's why the three 'uncertain' Justices were focusing on the Major Questions Doctrine. Given the nature of the IEEPA, and the then-recent dispute with Nixon over unilaterally imposing tariffs without Congressional approval, they believe it to be questionable that Congress would have "hidden an elephant in a mousehole" by giving him the power to impose tariffs without ever mentioning the word tariff (or excise or impost or duty or any other synonym). Congress knows how to talk about tariffs, and they have adopted lots of statutes that deal with tariffs - if they wanted IEEPA to include tariff authority, would they have "hidden" that by using the word regulation and not say the word tariff?

More importantly, does SCOTUS - do Republicans - want it to be the case that every time a statute gives the Executive the ability to regulate something, that automatically includes the power to instead impose new taxes on that something? That the President can on his own just enact sweeping tax increases by taking every instance where he or an agency is given the power to regulate something, and then say that he's going to "regulate" it by imposing a big old tax on it?

Given the Court's current support of the MQD, I think the more likely outcome is that the Administration loses. You might see a plurality opinion, with five or six (depending on Gorsuch) agreeing that IEEPA doesn't include the power to impose tariffs, but with a split between the three liberals and the two-three conservatives over whether the MQD is a necessary part of the analysis. Maybe Gorsuch concurs separately so he can bring in his NDD stuff.