Iran has about as much of a reason to build a dirty bomb as Assad had to "gas" his own people.
It is filled with intrigue, that's for sure.
I think that if your idea is true, the dirty bomb would be set off in Iran, and the cover story would have been that Iran was making it or transporting it illegally. They would have gotten a two-fer out of it. Seems plausible if Hilldog was elected.