Even as this story falls off the front page, I want to quickly come back at you, because I like your perspective. I haven't been following the public statements from the White House about ACTA. I should look for that.
Let's set aside the constitutionality of an ACTA signed and executed on the sole authority of the president, because I think we agree on that. In real political terms, I don't think that the current illegitimate or half-way legitimate status of the agreement/treaty as it stands is good for the interests of either opponents or those in favour of ACTA.
One important purpose of ACTA seems to be to promote a new set of international standards for the enforcement of intellectual property law. That purpose is undermined if the US seems to be uncommitted to it. Similarly, if ACTA is supposed to provide a more favourable, more exclusive venue for development of international IP law than WIPO and the WTO, another apparent goal of the process, the US doesn't advance that objective by undermining the treaty's domestic legitimacy. ACTA as some sort of variation of a sole executive agreement isn't good for ACTA partisans. When you're looking for international agreement, you don't want to be splitting legal hairs.
The current status of ACTA in the US isn't good for ACTA opponents either. For one thing, ACTA is at least superficially in effect, or will be, so the main goal of opponents seems to have failed. Even if the ACTA signing were to be declared unconstitutional in America, however, there remains the question of international legitimacy. The fact of the USA having signed ACTA puts the state under an international obligation to abide by it. That obligation has unpredictable effects and cannot be dismissed as an irrelevant imposition on a sovereign country. It can affect other US interests. It complicates opposition to the treaty through rhetoric and legal argument inside the US. That's not quite as strongly put as I'd like, but I can't spend a lot of time on it.
I suppose it's possible that the administration is playing some very deep political game. It could be trying to undermine the international IP regime, but I don't believe it is, at least not without instituting a modified replacement, which is what I think ACTA is supposed to be. It could be trying to leave the status of ACTA uncertain so that the TPP can eventually supersede it as the standard for international IP enforcement. That strikes me as superficially more likely, but still a bit paranoid.
I agree that the administration is overstepping its constitutional authority. The questions that come up after that are interesting. Why is it doing that? What does that choice reveal about the interests of the ACTA partisans in the US? Who is that choice good for? I think it's basically bad for everybody. I think it's a political and legal bad call. The best would have been if ACTA had gotten its day in the sun and been rejected. However even if it had been accepted by the legislature, that would have been a better situation than the one we find ourselves in.