Our ability to restrain ourselves is underrated. We are the only nation to have used nukes and we will be loath to repeat that against a civilian population. That said, you are correct if a high megaton weapon is used against or population. Odds that this will be the case are low. A single 10 to 20 kt weapon though devastating will probably not trigger a massive response unless a nation state is directly implicated. I have my doubts that anyone would do it that way. Probably a surrogate would do the dirty deed and have sufficient cover to allow the leftists here to say: "are you sure?" and "slam dunk huh?".
Both rational and arguable and I waffle between your position and the one I expressed. The above has been my default position for many years. However, I've grown MUCH more cynical since the Dubai ports lemming stampede.
Shep Smith and Aaron Brown will hype the footage of even a "tiny" nuke (as if a nuke can be tiny) and the sheeple will demand blood. Personally I'd rather nuke Mecca than have a herd of murderous no-nothing sheep choose to slaughter their neighbor (or the guy at the 7-11) because they thought they might be a Muslim.
Here's a thought. If we nuke Mecca we'll "piss off" many who already want us dead and maybe a few who might let us live as dhimmi, but I bet there will also be a pot load of Muslims who will blame Bin Laden and the Iranian Mullahs. And they're much more accessible as an outlet for their anger...
At times my cynicism gets the better of me. I'm probably leaning more towards "nuke 'em till they glow" right now partly because I'm in the middle of reading Thucydides History of the Peloponnesian War (for the first time in 35 years - and then it was only excerpts, now I'm reading the whole thing)
As to plausible deniability... I don't think anyone will get the opportunity to raise the issue and, if anyone does, I wouldn't want to be standing near them when they do such a foolish thing.
Niven's Laws number 1A
Never throw s**t at an armed man
Niven's Laws number 1B
Never stand next to someone throwing s**t at an armed man