I'm a bit confused, the credits and Snapchat show humans and trolls living on Earth c but in Calliope and caliborn's back story, it showed alternia. Did the trolls just happen to find a planet identical to their home? Also does it matter the the father cherub destroyed alteria c as well?

It didn’t show Alternia, it showed a planet populated with Trolls that the Cali twins’ father attacked. When their mother caught up to him they fought and she won. He then slithered off and laid an egg on far off future Earth.