Quote: "Can you please clarify your answer regarding the first question ?"
you st collison as you would for any other object, there's no difference between objects that are part of occlusion system and the ones that are not
Quote: "Can Dark Imposter objects render shadows while the imposter system is active on the 3D objects ? "
i have no idea, i would say they can't. When object are in the distance they get turned into images - there's no pint in having shadows for objects that are far from the camera. When they are close they are just regular objects. Imposters get activated on the objects in the distance only.
Quote: "Can shader effects be assigned to Dark Imposter objects while the imposter system is active on the 3D objects ? "
yes