How does simultaneous combat work in Civilization 5?
"Simultaneous" in this context doesn't mean everything happens at the same time, it means every player takes their turn at the same time (not sure about the AIs...), pretty much with the rules you'd expect in single player - if you tell a scout to move, it will move straight away.
So, in your scout sceanrio two things could happen:
You move first: Your scout moves away, the archers cannot fire because they are out of range. The archers are still free to move as usual.
They move first: The archers bombard your scout. Afterwards you can still move your scout (assuming it is still alive).
I've not played a lot of multiplayer, and haven't had a really intensive war yet, but I imagine there's a little bit of "fastest trigger gets an advantage" in situations...