MY THEORY ABOUT THE HIGH SPARROW IS...
He's probably a pedo-bear.

(Just like Meryn Trant was.)
That's why Qubyrn tells Cersei that theres
'much more' to all the rumors....
That's why Qubyrn also tells Cersei that his
'little birds' can confirm it....
Cersei already KNOWS about all the 'caches' of wildfire hidden all around the city.... because she HERSELF was adding to the big stockpile back in season 2, right before the battle of Blackwater.
When Tyrion was 'Hand of the King' he told the Old Pyromancer "you won't be making this stuff for my sister anymore... you from now on... You will be making it for ME instead."
So, the High Sparrow is probably 'blackmailing' Tommen with all shit they have done. (Or getting ready to blackmail him

)
That's... WHY... he is able to control Tommen to such an extreme extent that he make all these decisions that go against his family & gives them a death sentence.
That's WHY a sex-pot like Queen Marjorie is unable to get Tommen aroused enough to make a baby... But she lied to the HS and blamed herself... To keep Tommen's name clean, just like she protected Renly Baratheon about the rumors.
"Do what The Gods say (through me, of course)..... or I will expose you"
High Sparrow gonna get caught red-handed.
Feeling deeply ashamed... Tommen prolly gonna kill himself, out of pure embarrassment.
Then Cersei gonna try to blow everyone in Kings Landing to Kingdom Come with the Wildfire, as payback.
Just my theory tho.
I could be wrong about all this shit.