I'll confess it's been years since I read the books so I can't debate you on specifics. I recall enough to be able to selectively google though, and it seems like fAegon was at least thought of as a possibility in GRRM's original plans; there's the mummer's dragon reference in the HotU:
... what does any of it mean Khaleesi? A mummer's dragon, you said. What is a mummer's dragon, pray?"
"A cloth dragon on poles," Dany explained. "Mummers use them in their follies, to give the heroes something to fight."
And there are repeated references throughout the early books to Clegane dashing Aegon's head against the wall, which in itself seems like foreshadowing.
Ultimately though, as someone who has dabbled in writing before, I know that a lot of it is just written on-the-fly. GRRM seems to be able to plan and foresee where he's going better than most, but I wouldn't be surprised if fAegon is just something he put in the later books. It's not sloppy, that's how books are written. Look at how Tolkien changed the Golem encounter in later editions of the Hobbit.