I think the West has already demonstrated that they're perfectly happy to just ignore obligations like that, as evidenced by them all refusing to inforce the arrest warrant against Netanyahu.
Forced by who? The Republican Congress would likely say Iran deserved it, and even if they didn't Trump would dismantle any group the executive branch is supposed to use to enforce them as he was pushing for with Russia .
Their biggest trading partner is China .. not sure what they would do
Why would you think they would need to be sanctioned for not using them? China and India both have nuclear weapons and have small skirmishes (granted not as big as this) and we don't discuss sanctioning both of them for it. I would think threatening to use or using them would be the only scenarios where sanctions would be "forced hand" for lack of a better term.
I saw elsewhere that Pakistan stated they would be attacking Israel back with nukes if Israel used them against Iran. Which is why I assume it's a given they won't be used and we won't have to worry about them coming into play