That’s the reason. Dolphins also pick on fish just because it entertains them.

I wouldn't put it past them

Hank Green did a video about Sunfish and apparently they are mostly collagen which is not very nutritious so most things don't bother eating or hunting them so it's quite possible the orcas just do it for fun. I think the article speculates that breaking the Sunfish into small pieces might help young orcas to eat it more easily though so it's a bit unknown I think

Probably. Orcas can behave in quite a human manner sometimes.