Very good questions, Cubibobi.
1: ‘Nature’ or Existence or Reality works within the framework of eternity. To the human mind, 30 mahakappas are a very long time. But the Buddha said that all the tears we have shed, or the blood we have shed having been slain as animals FROM BEGININGLESS TIME is more than the waters of all the oceans; and that is difficult to come across ANYBODY in life who had not been your mother or father or brother or sister etc. And those are only analogies, to give some idea, a very vague one ,to His listeners. In reality anything that has no beginning will admit of no fitting analogy, only one that is beginingless as well.
So however long it takes to be born a human in a Buddha sasana on this planet (among the many inhabited ones in this cakkavala) is still a VERY SHORT TIME when one knows that once Nibbana is attained ,that state will then last FOREVER.
2: The Budddha himself said that with time the Dhamma would disappear, gradually. And where there is no Dhamma there can be no Sangha to spread that Dhamma. Now how the vestiges of the Dhamma in Kassapa.s sasana came into the hands of the Hindus in the next (Gotama’s) sasana, I do not know.
3. No, apparently not. Only devas and Brahmas can. For if it were otherwise, those gandhabbas from other inhabited planets in this cakkkavala, or some of them at least, would have come to listen to the Buddha as well, but the Tipitaka makes no mention of them.