The red witches have been known to be wrong and change their allegiances
The battle outside Kings Landing will be eye candy. The real battle will be inside when Arya and The Hound infiltrate the city and extract their vengeance
After Arya strikes another name (or two) from her list and we are finally blessed with Clegane Bowl, the Dany/Sansa rift opens setting up the final episode.
Dany does the nutty Targaryian thing forcing Jon to side with his Stark cousins over his Targaryian aunt. When Jon's true lineage is revealed he abdicates and leaves the 7 kingdoms to rule themselves.
Then he goes to find Ghost and apologize. That was a really dick move in Episode 4 to just give up his dog without even saying good bye
Varys seemed to have been on good terms with Melisandre who you should remember was able to hide her true form. Perhaps the Lord of Light has been shrouding Varys?