Eh. That's really not a realistic fear here. The US already has nuclear weapons assets within striking distance of Iran. Sure, ICBMS launched from North Dakota would have to fly over Russia and risk triggering a false retaliation. That's why the US simply wouldn't use those launchers. Nukes launched against Iran would be launched from submarines in the Indian Ocean or Mediterranean or be delivered via cruise missiles and other stand-off munitions. They wouldn't fly over any nuclear armed state, except maybe Israel. And Israel isn't particularly worried about being nuked by the US.

And no, this wouldn't trigger a retaliatory attack from the other nuclear powers. The launches would not be falsely read as nuclear strikes. The best example of this? Consider the Tomahawk missiles. The US has already launched hundreds of these things against Iran, and it didn't cause Russia or China to press the button. And the Tomahawks can carry both conventional or nuclear warheads. They could destroy all of Iran's cities using nuclear-tipped Tomahawks, and the launches would look no different than the hundreds of launches the US has already used against Iran.

This is still a horrible idea, and horrible doesn't even begin to describe it. We're talking about an act of mass murder that exceeds the Holocaust. That's reason enough to consider the idea completely unthinkable. But it should be opposed for the sake of the Iranian people, not out of some unrealistic fear that it would trigger a global thermonuclear war.