I think the papers you named are sufficient.
You should perhaps xerox your passports just in case you lose them. And then of course not keep the copies at the same place as the original documents.
Might help when you have to contact the embassy of your country (where are you from?).
For your real important stuff such as flight tickets, passports etc there are lockers at the festival which you can use to protect your stuff. It's not really expensive.
Regarding the second question: When you're 16 you are allowed to buy beer and wine (including mead
) in Germany, but not the stronger stuff such as vodka, whisky etc. You should take your ID or something with you when you buy alcoholic beverages in the Wacken supermarkets (in the village) just in case somebody asks you. Which is happening more and more often nowadays as many young people in Germany tend to illicitly buy alcohol and then end up in hospitals.
On the festival itself people are much too busy to ask you for your age. Tell them what you want and you'll get it.
As long as you don't look like you're 12.
And no, although you are officially not allowed to stay on the actual festival area when you're 16 or 17, they won't shove you off the festival area at midnight.
Nobody's really controlling that. If you want to play safe just don't leave the festival area at night until you really want to go back to the campground.