The point is, it's still not Emmeryn, not at any point. It's on par with Valdiar being the good guy and trying to stop Grima from destorying the world, it's extremely out of charater for what Emmeryn is established to be like in Awakening and would be a serious tonal whiplash besides.
You've got to remember that this is a child who hates violance as a woman as shown in the comics that accompany the game as well as in game when Chrom tells you the player that Emmeryn was willing to let her own people throw insults and stones at her without retaliation. DOES THAT SOUND LIKE SOMEONE WHO WOULD KILL THEIR OWN FATHER?!
Also, conversely are you saying that Chrom lied about his sister?! That she wouldn't be able to bring the degree of peace that she did by not being her father's murderer?!
I doubt that Ylisse would let her live if she was the one who killed her father. It's highly implied that Chrom's father died on the battlefield or from his wounds due to infection. You don't at ten years old have the capacity to kill your own parents, even in a volitile environment like Emmeryn was exposed to.
I'd probably want to see more about the formation of the Grimleal, or at least more of Plegia's culture if they're going into the actions of Chrom's father and his impact on both countries, probably playing it from Plegia's perspective and getting to know Gangrel from how his own people saw him. Heck we could even have a name for Robin's mother, and see through her eyes what made her want to leave the Grimleal.
Just a thought. Hell, I've got it on my backlog of stories to write anyway.