I think Israel is going to use a tactical nuke to shut down Iran's program for good. It's the only way they can destroy the underground facility. This will be blessed (if reluctantly) by the United States, and blessed (if very privately and even more reluctantly) by Russia.
The bottom line here is that Iran is getting "too big for its britches." Russia and China are supportive of any country that opposes America--if Hitler came back from the grave, Moscow and Beijing would welcome him with open arms...at least until he began to amass a war machine big enough to threaten them. Russia and China don't mind an Iran big enough to cause trouble for America and the rest of the world.
But a nuclear-armed Iran? That's a different story. That would make Iran--at least in one respect--a peer of Russia and China. And that's not something Russia and China want. Especially when that nation's leaders make clear their loyalty is to Islam first. Both Russia and China have problems with militant Islam. So they've got to be asking themselves if the same venom Iran has toward Jews could someday be projected at Russians and Chinese--I mean, none of them bow to Mecca five times a day, so all are infidels and Iran's brand of Islam says all infidels and their women are fair game for jihad.
Add to this the lesson Russia is learning in Syria: sometimes psychotic despots are more trouble than they're worth, even if they do hate America. Russia has taken quite a beating, in terms of regional pr, by going all-in on Assad. And after all that, Assad is still likely to fall, and whoever replaces him is going to remember Russia's support for their oppressors, and probably cancel Russia's lease on the naval base there...or at least demand a lot more in return for the privilege.
So I think what is going to happen is Israel (possibly with American aid) is going to drop a tactical nuke on Iran's facilities. When the radioactivity gets detected, the Israeli response will be a rolling of the eyes and something along the lines of "well, of course there's radioactivity, they had tons of highly enriched uranium there, just like we've been saying all along, and of course it's going to leave a mark." The corporate media here will hush up Israel's use of the nuclear option. Russia and China will protest. Privately, Russia will tell America that its willingness to not raise a big stink about the use of nukes should in no way, shape or form be construed as consent for America to effect "regime change" in Iran or invade, and that this is a one-time, not-to-be-repeated loophole in policy re the use of atomics. And America will nod and say we get it. And that will pretty much be that.
Iran, for its part, will realize it has reached the end of its leash. Its leaders will engage in typical Islamic fulminations at the mouth about Jews and infidels and evil-doers...while privately negotiating a compromise that will allow it to save a little face while ceasing its attempts to acquire nuclear weapons.