> I think you're right. If I'm sitting on the court, I might accept whatever flimsy loophole the Executive Branch provides, and say, "Congress is free to clarify their intentions at any time to resolve this impasse".

See, that's why you aren't on the Court.

What the kind of people that actually get put on the Court would mostly due to allow the action without endorsing its legality would be:

(1) Be sticklers about standing and other threshold issues (ripeness, mootness, etc, but standing is the big and most useful one here); almost no litigant other than Congress acting as a whole will have the kind of particularized injury to allege that would give them standing to challenge paying the debt (an actual creditor might have standing the other way, but that's not the scenario we’re concerned with.)

(2) If you really can’t kick the case for standing or other threshold issues (well, first, amateur, but...) then invoke the “political question doctrine” to avoid it anyway.