Statutes can delegate the president tariff powers and appropriately circumscribe them, and that's what both the IEEPA and Trade Expansion Act did - they gave the executive tariff authority in certain circumstances.

Trump is stretching the definition of those circumstances to the limit, in my opinion, especially because protectionism is the fundamental policy being pursued. If we want general protectionist tariffs than the law should be passed that enables the executive to do so, but that hasn't been forthcoming.

How effective are tariffs as a trade negotiation tool if your counterparty believes its likely the courts will take away the big stick?

There is no reason for tariff policy to be made quickly. Indeed, it would be much more effective if it could change only very slowly, because any positive effects tariffs may have rely on businesses having certainty when making decisions with multi year horizons. Only a fool would make a decision about where to build a new factory based on the Trump era of whim based tariff policy.