The Iran deal will not be defeated by Congress. The President has enough votes to block an over-ride of his veto, should it prove necessary, and I do not think it will prove necessary.
The devil is in the details to be sure. But on first blush I think the deal is better than the absence thereof. The US is not about to bomb Iran, and Israel probably isn’t willing to do so either, certainly not on its own.
Could Iran become a nuclear weapons state in 15 years? Yes, but probably not before then, and steps can be taken in the interim to interdict them, even by military means if intelligence shows they are preparing to break out.
Also, the real sweetener is the 5 year restriction on the import/export of conventional weapons and the 8 year restriction on the importing of missiles or missile technology.
Also re snap-back of sanctions, the fact that a majority of the 8 member states with supervisory status over the deal can decide on snapping back of the sanctions, means that Iran, Russia and China cannot act in concert to protect Iran from the sanctions snapping back if Iran seems to be violating the letter or the spirit of the deal.
Finally, an agreement isn’t a suicide pact, and so if it turns out that there is real evidence that Iran is cheating or violating the terms or the spirit of the agreement, then the US and its allies, or the US and Israel, or Israel alone, will act, by one means or another, to bring Iran into line or under heel.
Nobody is going to trust in the eyes of the IAEA alone, and the US and the Israelis will certainly be using their own national technical means to keep a very close eye on what is really happening.
And just maybe the Iranians will come to change their tune and will accept that the downside of aiming to become a nuclear power is greater than the upside. I am not sanguine about this, and do expect that in 15 years or so, we will see the Iranians trying to get the nukes that they rightfully believe to be the ultimate guarantor of the longevity of their regime.
But if the Iranians move in this direction, there will be evidence of this effort, and steps to counteract Iran’s effort can and will be taken. Or that is my not entirely unreasonable hope.