Grease - please somebody finally explain to me why...

[Badtasteflump]

Does Sonny drag Sandy off the dance floor so Danny can finish the dance with ChaCha? It can’t be because Sandy wasn’t good enough as she & Danny had already won (they were the last couple still dancing).

And why does the car fly at the end? Google tells me one theory is that Sandy drowned at the beach at the start of the film and it was all a dream, but that can’t really be the plot can it?

I don't know why Chacha gets left on the dance floor unless it's to piss off Leo because Rizzo brings him in retaliation to Kenickie?

I do know that any film with a neatly-packaged ending like Grease is essentially a fairytale and what's a fairytale without a flying car?