[Agema]

Yes, it would be nice if a unit that does a controlled retreat from a battle didn't scatter. However, with a bit of preparation you can leave commanders in all provinces the army can flee to, or just have enough commanders in battle so that random chance distributes them to all necessary provinces. In theory you should be able to pick them all up again and send them in for more hit-and-run attacks, although they might get very piecemeal and it will become steadily harder if they scatter over more provinces.

I'm not sure if I've got the mechanics right on this but...

I think the movement order is semi-randomised. If you had 5 armies with fire and flee attacking an army trying to move into your territory, you should have a good chance of engaging that army in the province it is coming from as at least one of your armies hopefully should move before the target army. Therefore it won't go anywhere. If he can't kill your archers fast enough, potentially you can stop the enemy army and do plenty of damage to it for a few turns.