If Iran does close Hormuz, they will get everyone and I mean everyone coming down on them like a ton of bricks. Russia, China, Japan, Europe, America, Saudi Arabia. NATO. Quasi-government entities like the Bilderbergs (who wield more soft power than any government, including America itself). They would find themselves very quickly isolated from the entire planet, save for Al-Quaeda and the Syrian government. They would, within an hour or two, get phone calls from Russia and China telling them that if the blockade were not lifted immediately, the Tehran regime would be left to twist in the wind when American retaliation came.
As for nukes, pretty much the same scenario. America and Israel would likely be (very quietly) granted a limited, one-time not-to-be-repeated pass by other superpowers to use small, tactical nukes for the purpose, and only for the express purpose, of taking out Iran's nuclear program. This would fall under the heading of "preventing unstable wacko nutjobs from joining the exclusive nuclear club" much more than protecting Israel, under the calculus of other superpowers. Any other use of nukes, by America or anyone else, would still be strictly forbidden. So if Iran did lob a nuke at anyone, it would get four or five in return; they'd be fair game.