In a way he still was, but it was under the orders of Diavolo, not his own. He certainly is wondering how Diavolo was doing without him there to take some of the work load off him. "Yes, Lord Diavolo is the future king of the Devildom. His father is current in a self-induced slumber and has left his son to act in his place."

As for that other question, "Demons are not immortal, our lives are just far longer than that of humans. If something does not kill us, we will eventually die of old age." Which... is something that actually concerns Lucifer. Not for death itself, but where he will end up. The idea of being dragged back to the Celestial Realm... was hell for him.

Then that slight smile again, "Is this all helpful to you?"