Does this count as a breach of some sort of constitutional duty on behalf of the President? Or would it just be a valid argument that the "treaty" is null and void?
Sorry - not intimatey familiar with the US government.
No, that's the beauty of the thing -- if you can call a shitstorm on net freedom beautiful.
Usually when the legislative and executive branches get crossed up, the courts sort it out. But in this case Congress (the Senate) already ceded these powers to the executive branch under some other treaties. So there's really nothing to sort out. Congress (the House of Representatives and Issa) could make a big fuss and cause the administration not to implement ACTA, but it'd just be a temporary measure. You'd have to have a law passed specifically restricting the Executive branch from interfering with internet rights via previously existing treaties in order to really fix this thing. A long shot at best. You'd probably have more luck trying to teach beavers to fly airplanes than actually straighten out this situation "correctly."
Internet freedom, including privacy, anonymity, access, and publishing rights, are in desperate need of a constitutional amendment. Nothing else is going to work. There's simply too many structural avenues for attack by vested corporate interests.
And for purely political reasons, you will NEVER see Congress as a whole come out against the President like that. For example, back when Obama (illegally) attacked Libya, our Congress wouldn't even formally condemn Obama's actions.
The Democrats won't put the brakes on it because it would interfere with the power they're currently enjoying. And the Republicans won't do it because they too covet that power after the next election.
Sorry - not intimatey familiar with the US government.