I wonder how much of that nitrate is in Hezbollah’s hands and therefore accessible to Iran? Might very well have had some of our own troops or allies killed by bombs made with the missing nitrate. Foreign aid payments have been shown to be subject to avarice and corruption, and so the trend is to fund specific projects with deliverables and measurable success rates. There’s no reason we should be sending this stuff to a country like Lebanon given the political climate in that country for the better part or seven decades.
Nation states have no problem securing the raw precursors for explosives. The idea that Iran need some complex scheme is a little far fetched. Hezbollah, perhaps.