"Most importantly, basically any person who is an Islamic religious authority by training can interpret the Quran as he likes, and if he has enough followers, he can seek to force that interpretation on others (as al Bahgdadi is now doing, as Khomeini did, etc etc etc)"
Lodiac. That is exactly the major problem that we deal with and the reason why there is so much external and even more internal division and conflict in this religion.
Most other major religions appear to have multi-layer formal internal hierarchies (both individual and organisational) that ultimately point to a senior religious leader e.g. the Pope and seniors around him, Archbishop of Canterbury and seniors etc. There is also extensive formal training required to reach key levels. There is a certain degree of checks and balances internally that controls and limits the extent of individual radical breakaways and their propensity to cause damage in the name of the religion.
By contrast, Islam is a religion with fairly flat hierarchical structure and further, from what I understand, many Imams and some Sheikhs have little formal training - which is why we often hear the term "self proclaimed" Sheikh or Imam associated with the many dramas that we hear about in the news - that is one of the the core issues. Further, these individuals can craft their message however they wish and there is little control over this by any larger communal leaderships. Finally, combine this degree of power with the concept of no separation between religion and politics and you have a recipe for disaster - one needs only to look at the historical powder keg of relations that began the split between Sunni and Shia and the rift that continues to this day at a communal right through to a country level.
