"It is a screenplay written by a couple of kids high as a kite ... It is trash!"

"... Iran has an embassy in Mexico City.We are asked to believe that Iran and its skilled covert action force chose to set up an operation in which they ("they" being members of the "Quds Force" with approvals reaching up to Ayatollah Khamenei) relied on a used car salesman from Texas to recruit Mexican drug thugs to kill Adil al-Jubeir.  This is a problem involving the suspension of belief.  Why on earth would they create a situation in which they had to rely on this untested, untrained, unguided, and uncontrolled asset rather than their own people?

If I had been running this thing, (we're talking fantasy football here) I would have put a non-official cover operative or two (NOCs) into Mexico with difficult to trace communicatons to the embassy.  A senior official of the Quds force could be put on temporary cover in the embassy to control them.  You could cover the NOCs as tourists or business people probably with non-Iranian passports and legends.  You could have them keep their distance from your diplomatic mission while still meeting them clandestinely to vet progress and give instructions.  You could plausibly deny the NOCs if they were exposed in their covert activities.

None of this is "rocket science."  If as FBI Mueller said, this "plot" is like a Hollywood screenplay, then it is a screenplay written by a couple of kids high as a kite on a weekend and pitched in a producer's cottage on a film company's "campus."  It is trash.
The overwhelming likelihood is that this is someone's "information operation" intended to condition public attitudes for some purpose.  The over riding question is that of where the ovens are located in which this confection was baked and who the bakers might be.
Was it the Iranians as a subtle threat of serious action?
Was it the Israelis? 
Was it some combination of "rogue" American entities?..."