Ok that all sounds good.
I've seen both friendly and enemy corps pull back and form defensive lines, so I know it happens.
Once the corps is in contact, the regimental AI takes precedence over the corps commander's orders. This is correct behavior, as once you send troops forward you lose control over them. It makes it difficult to disengage unless you have another corps helping.
If your own corps are having problems forming a defensive line, it may be that you're trying it too close to the enemy. I haven't had any problems doing it farther back. It's probably that thing with the regimental AI again.
Hook