Sure. But having a closed source game does not prevent it from ending up on a warez site.
Yes, but it makes it harder for the hacker. You'd be practically handing it to them on a silver plate.
No. Open-source games, like open-source operating systems are open to scrutiny.
I was basically repeating what Darkheart mentioned. Open-source software can be improved or abused, in the case of games it is more likely to be abused.
But seriously if you're afraid of a little competition from some week-end coder, your chances of turning a profit on your software is marginal.
Don't underestimate the week-end coder, some of the best coders are self-taught. It's hard enough to sell a game today without helping the competition.
You're not *likely* to make much money with non-GPL code either.
If you've written a really good game you're likely to make some money out of it, but if it's open-source you're probably not going to make any money at all.