The codex says a human kill her and they found their body, only that, they dont even know who killed her.
They were invading a foreign country, they had no right to kill her.
Because it was the nearest human settlement, how they would invade Orlais without passing by it?
The author of the codex doesn't go into detail about exactly how they know but it does say explicitly the Chantry killed her. If you assume he's unreliable about this, you might as well throw out the whole codex. But you have no reason to think he's wrong.
"As always, their Chantry was swift to spread lies. In their haste and anger, they killed Siona's sister for wandering too near the hunter's path."
For the last time, the elves didn't go there to invade Red Crossing, they went to bring one of their own back. Besides, are you saying if you are in a foreign country you have no right to defend yourself?
The border Dales-Orlesian border is pretty big, why would an army be forced to go through Red Crossing?
And regarding Drakon, I was referring to the Daughters of Song codex found in the Fallow Mire. They were a hedonistic Andrastian Cult that was destroyed in Drakon's unification of Orlais. At one point they "numbered in the thousands". When the Empire's armies attacked their village, they refused to fight back.