1. I think in this sentence they are probably referring to some sweets that normally come in a bag. So it you have a bag of jelly beans, this flavour might be the king of the flavours that you get in that bag.
2. I'm not sure - I think in the Cursed Child, because it is a play, they are stage directions so it may be for the actors to decide what the 'something' is.
3. 'Pop' is a common colloquial word normally for 'put'. So 'He pops sweets into his mouth' = 'he puts sweets into his mouth' or 'I'll pop the kettle on' = 'I'll put the kettle on'. You can also use 'pop' to mean go quickly and then come back so 'I'm popping out for a bit' would mean 'I'm going out, I'll be back quite soon'.