How is movement determined?

[thejeff]

Assassinations do happen first. Catching sneaking troops happen after other battles.

Siege breaking happens along with movement, since reinforcements for either side can arrive before the battle. The besieger pulling out to friendly territory happens first. Invading a different province should work normally.

I'd assume stealthy units attacking don't interfere with movement. In your example the defending army would move first, since it's friendly movement, then the scout would attack. I've seen this happen, using scouts to take 0PD provinces from the AI.