I believe you are allocated a set number of tickets which are sold to away fans by the club. The number depends on the away club's reputation and fanbase size.
You don't get any gate receipts from playing away. Your fans are allocated some parts of the stadium to be in there but all the money go to the home team.
It depends. It could be between 5-15% at some grounds.
Either way you don't get very much money. I don't know what teams are in the Championship on your game but if a team like Newcastle are, there's a good chance