1) currently you can only create queries on properties of a single entity, so you can say that you want to get the Wheel entity where the revision is 456 and the wheel.car_id=123. You *cannot* refer to car properties, e.g. say that you want wheel.car.color="red".
But yet, if relations worked normally, then you would be able to view the database as it existed at a given revision.
2) No estimates . It is only possible for the validity audit strategy.