The Iranians are not totally stupid, they could given time find fissile material that could be traced back in some fashion as being from the USA, they could IMHO very much attempt an attack upon Mecca, maybe an aborted but successful detonation nearby or just the discovery, just what it would take to awaken all the latent moderate Moslems in America.
All the world believes its a CIA attempt and thus like throwing rocks at your neighbors hornets nest above his open window who will the hornets attack?
You miss the point. The Iranians want Mecca as it is, intact with no blemishes.
Mecca is the most holy of hollies and the reason for being Moslem.
Never in their wildest dreams would the y consider harming, much less destroying the site of the pilgrimage