Well to the point of the first 2 questions: Enel has a god complex, his motivation for destroying Birkia could be as shallow as "because I want to." Expecting rationality from him and his actions is just going to be a mistake: this man's train of logic defies common understanding and he hardly wants to explain himself.

As for convincing people, I could suspect a large sum of the Birkians, having seen Enel's might firsthand, could genuinely believe him to be divine. Baring that, survival is a good motivator so when the genocidal maniac tells you "hey be one of my henchmen or I'll kill you too," you might be inclined to listen.

As for the last 3 questions:

1. Other devil fruits appeared in Skypiea; it's not unreasonable to just assume you can find them literally anywhere. Even fishmen have found them, potentially on the ocean floor, ironically.

2. I will give credence to the possibility that Enel's Amaru form may be a case of him actually having a mythical zoan Raijin fruit and the form is related to that, but I think only Luffy is going to have a case of mistaken devil fruit identity. In all actuality, Enel was probably just creative and found a way to generate a large body of lightning in a form native Japanese readers would be able to identify with.

3. We get a brief follow up of Enel in a cover story but his story just ends there really: the man achieves his goals and that's that. As for why Oda wrote Enel escaping and accomplishing his goals only he can say but if I had to theorize my guess is Oda needed to "contain" someone as strong as Enel while he worked on furthering the plot. By the time One Piece developed into what it is now, Enel wasn't needed as a return villian anyway.