Re: ball - you won't be able to go to the bar or Formal that night as most of college is off-limits to those without tickets. Not quite sure how it works if you do want to stay in your room... I think they make you leave for a few hours around the beginning of the ball but you'd have to ask someone on the ball committee. Of course it's a lot of money if you won't enjoy yourself, but if you're just not sure it may well be worth giving it a try (no I'm not on the ball committee and selling it, this is just what I think). It will take up the whole of college, you'll be able to hear it from your room so getting work done won't be easy, and it will (for most people) be the only one that happens in your Oxford career.
Of course do what you like, but I'd wait til I got here before ruling it out maybe.