The problem here arises when we say that they are all valid ways, and we then go on to say that prayed behind such and such a person is void. Do you seriously think that Imam Shafi'i while holding the opinion that Basmala is Fard in Fatiha wouldn't pray behind Imam Malik who beleived it is Makruh. As long as it's opinions based on sound understanding and knowledge, and not whims and desires. Yes we may find such fatwas in some books, but that doesn't mean they are necessarily correct. Just as major Imams have said that one can't marry outside the school etc. Such extremism wasn't uncommon at certain times in Islamic history. Let us not repeat their mistakes.