Sounds a bit unfair to all the saffron farmers in other countries. With half the land in Afghanistan suddenly devoted to saffron production, the value of saffron will plummet. Meanwhile, Afghan saffron farmers will be selling their saffron to the US government for some insane price. If I were a foreign saffron farmer I'd be pretty pissed off. Oh, and the number one saffron producer at the moment is Iran, so you'd probably get a vast industry in smuggling Iranian saffron across the border to be sold in Afghanistan.
Oh, and also the drop in opium production will raise opium prices, and so you'll need to keep raising the Afghanistan saffron buying price in order to keep it more attractive than opium farming.
I foresee all sorts of unforeseen consequences, and those are just the foreseen ones!
I agree with the problems you mention, and the likely unintended effects. My point was that it might be a least-worst-option alternative. I suspect there are no perfect solutions that can be imposed onto the Afghan people from outside.
If the could grow corn or soybeans - which are the main feedstocks for biodiesel - it would be better for everyone if they just used it for food production, as opposed to refining it and shipping it over here.
Oh, and also the drop in opium production will raise opium prices, and so you'll need to keep raising the Afghanistan saffron buying price in order to keep it more attractive than opium farming.
I foresee all sorts of unforeseen consequences, and those are just the foreseen ones!