Hi Ryanj,
Welcome to our subforum. I'll try to answer your questions basing on information I have.
1. AFAIK, there's no penalty for re-applicants. However, the positive changes in your profile are desirable in such a case.
2. You'll need to do your apps from scratch next year. Perhaps you could ask for special consideration, but it's not a standard procedure.
3. I'm afraid that in general, yes. Some particular scholarships will still be available even for those who apply later (you'll need to check the sites to see which ones), but overall chances to get one certainly become weaker.
As to your more general question whether to apply or not - of course you know that well prepared app is better than rushed one. However, if you want just to play the lottery and see what will come out of it - I'd say why not. Your chances will probably be lower for Cambridge, as they consider the written part of your app very seriously (and are very selective, too), so the lack of preparation will hurt you more there. However, your academics are really strong, and they may still want to interview you as they are after something 'special'. And I think that you have a very good chance at Oxford - they care even more about academic strength.
Your age is below the average for both schools, but I don't think it will hurt you, providing that you meet their experience requirements. Also, the lack of community involvement is not a deadly flaw for Oxbridge: I've seen admits for people with no community/social activities at all (they did have other interesting hobbies, though - but so do you).
Btw, if you don't mind my asking, are Oxbridge your top priority schools or just 'why not?' ones? I read in your profile that you thought about LBS and MIT as well - so, what is more important for you: to get to a higher ranked school next year or try your luck with Oxbridge in R3/4R?