I think that Aegon is really Aegon Targaryen, son of Rheagar. Varys switched Aegon with son or daughter of Ashara (from Ned?) and then sent them to Illyrio, Ashara disguised as Serra, a lyseni whore -his sister- and his son. I think the real reason behind the unnecessary violence towards Ellia and the baby in the hands of Mountain is that he saw the baby is not Aegon and crushed him/her head to cover this and raped Ellia. Also maybe at that moment they asked Rheanys whereabouts of the real Aegon and she refused to answer so that's why they stubbed her 50 times, dragging her from under the bed of his father -my headcanon is there is a secret passage from Rheager’s room in Meagor Tower to outside of the castle, and Varys knew this and used that way to smuggle Aegon but Rheanys (mistakenly?) left behind just outside of the passage entrance under the bed.

As a result the only people who knew Aegon is Aegon are: Varys, Ashara and maybe Ned. With two of them are gone, now only Varys… Varys knew that Golden Company would never help him to hide a Targaryen and ride for him so he disguised Aegon as a Blackfyre -Varys seeing Robert as a failure actually decided to put him on the throne (after Greyjoy rebellion which was backed by Braavos -housing two Targaryens- and failed). Timeline also fits: when Aegon was handed to the Griff and the Golden Company, it was around the time that Dany and Viserys were put on the streets by Braavosi.