You bring up a number of valid points. I am sure you are aware to some degree as how the various sects of Islam ignore the teachings of each other. Wahhabist, Salafist just to name two, do not even think Shi'ites are muslims but apostate at best.
Many of the more radical sects/sub sects, offshoots would openly condemn say the Zikri sect founded in 15th century as not being worthy of walking on the earth. Islam is one big pile of mixed often highly agitated, hard headed, poorly educated accept within their limited circle, groups of peoples, often driven as much by various Islamic ideologies as by their particular ethnic/geographic locations. As you are well aware, some sects only follow the Qur'ran, and totally reject all that is in the various Hadith books.
So how can we expect anyone to even bother attempting what you suggest? There is much more divisions amoung many of the Islamic groups (including many modernized movements found in a given country) then there is divisions amoung say Christian sects. One would not expect an edict prounounced by the Roman Catholic Church would hold any effect in say the Church of Later Day Saints. Right? Your example though ably put regarding the Shi'ite Mullahs fatwah against Salman Rushdie was not recognized by all muslims as committing an offense. That is, the fatwa was issued by Shi'ites, and Sunni groups who don't even think Shi'ite are muslims just looked the other way. Some Muslim Scholars had followed up over time and said it is hard to find fault with what is plainly shown in the questionable verses in the Qur'an as being able to be defended. So I would ask you, why would those Muslims want to adhere to a fatwa issued by a sect that is not recognized as being Islamic.
The problem as partially iterated above is that Islam is a mix of various believes, and therefore one cannot expect all of them to fall under one broad leadership. Especially when they diverse in so many ways both in theology, ideology and ethnic lines.
Can any state committed to political Islam as it ruling ideology ever be a reliable U.S. ally, since the whole basis of that ideology lies in drawing sharp distinctions — beginning with what's permitted (halal) and what's forbidden (haram) — when much of what the U.S. represents falls on the wrong side of the line?