there are some nuances with the UNSC snapback timing
triggering the snapback means notifying the Joint Commission (JCPOA member states) that it believes Iran is in material non-compliance with its obligation. this alone does not automatically result in UNSC resolutions/sanctions being reimposed.
the next important step is for the invoking state(s) to notify the UNSC that it believes Iran is in material non-compliance with its obligations. theoretically, it can do this immediately after triggering snapback. this will trigger a 30 day period for the UNSC to pass a resolution to continue sanctions relief (this can be vetoed / blocked by the US).
if such a resolution is not passed within 30 days, all nuclear UNSC resolutions/sanctions against Iran 'snap back' into force. a strict reading of the text suggests that once referred to the UNSC, not even the invoking state can withdraw it (at this point Iran is doomed as the US will simply block any attempted resolution to continue sanctions relief).
however, the JCPOA also provides for mechanisms to resolve the triggering state's concerns with Iran's compliance, which could last up to 50 days. importantly, the complaining state is not required to pursue these mechanisms.
this means the Europeans can trigger snapback (notify the Joint Commission it thinks Iran is in material non compliance) without starting the 30 day clock, giving room to negotiations with Iran to reach a deal before it reaches that stage. however, the Europeans would have flexibility on whether and for how long to try to resolve these concerns with Iran. overall, this places Iran in a very unfortunate situation.
at this point, the best case scenario for Iran is that Russia and China may refuse to recognise the legitimacy of the snap back and de facto continue to trade with Iran. however, past experience teaches us that UNSC sanctions are a serious barrier to trade with Russia and China, and at best they remain a strong justification for those countries to refuse to do business with Iran.
@ShapurII