IF nukes were used - VERY DOUBTFUL - they'd be "low yield" and either be deep-ground penetrators for the production facilities that are buried very deeply OR a low-yield airburst high enough to create a localized EMP. The Iranians may have hardened most of their sites for this, though.
OH YEAH. If Israel does an effective job of setting back their nuke card, they'll send every asset they've got at Israel and they can do heavy, heavy damage to Israel with conventional ballistic missiles. The Israelis know this all too well and that's why IF we see them strike, it will be because they have no other option. Iran must not be allowed to have a weapon with which they can shut down the flow of a major portion of the world's energy flow and do so on a whim.
They've already floated (forgive the pun) the idea of charging every vessel through the Straits of Hormuz a "fee" for passage. Once they have an arsenal of nukes, who will be willing to test the mad bastards? I think Biden's top handler, the Tan Messiah, hates Israel enough to create this kind of threat to them. He's an EVIL bastard, himself.
That depends on what you mean by "involved". I doubt they'd field troops against Israel (for now) but they'd rally all their buds in the UN to make Israel's life as untenable as possible. I think the current DC regime would turn on Israel completely if they attacked "without permission".