The faith militant thing is one storyline I'd defend. We already saw long ago the people attack Joffrey and cause a riot. Remember that the 'idea' of power keeps the smallfolk in check. There's a total power vacuum in King's Landing right now with Tommen on the throne and armies decimated by years of war. We've also seen people preaching about the degradation of the royal family on the streets for quite some time, and religion has always been a big part of the show.
It's not at all beyond the realms of possibility that in desperate times of poverty and need that all these kids would sign up for a religion like this. And with Cersei's stupidity arming them, it's perfectly possible that they could stage an armed coup while the King is too weak to order the use of force. All of this has been fairly well signposted at one time or another. And with the show having examined so many different types of power, seeing a wholly (or holy, lol) religious type of power-wielding maniac is different, refreshing but in keeping with the theme of the show. I really like it.
It's not as if they're signing up to a new religion, either. The Faith of the Seven is the accepted (official, even?) religion. While I find the brothers' garb and fetish for branding their foreheads a bit extreme, that's really for the Sparrows. Also, something I picked up tonight: Cersei addressed the High Sparrow as "High Septon." In other words, and maybe I missed
when this happened, but the Sparrow isn't just the
de facto leader of the religion in King's Landing, but he's the
de jure leader too.
Besides, considering Tommen's proclivity to bang his wife (pre-imprisonment) and play with Ser Pounce, and Cersei not even managing to stay classy while day drinking, the people probably do respond well to otherwise decent folk giving them food and the shoes off their feet.