I think you can have a invisible "flock entity" that is the boss of the formation, and make so the other creatures position himself relative to this entity. If this entity can rotate, then the relative position will rotate too.
...oopos.. you are already doing that.
What about this:
Why you cant calculate new positions for the entities colliding? I don't need to be fancy, maybe just dram a line from the desired original point, and the leader, and move the desired position to the middle of this line. Test again for collision.