Moin,
I'll try to answer your questions as well as I can.
Travel light: it does cut the cost of course, but also keep in mind, that YOU have to carry it all around as well. So it's up to you, whether you take a tent or not. But tents nowadays weigh less than 2 kilos so that is the least of your worries, I reckon.
Lockers at the airport: I honestly don't know if there are any! But there are lockers at all major railway stations (Hamburg Hbf/Main and Hamburg Altona) However, I don't know if they are limited in time. Check out
www.bahn.com for information about the stations.
Camping: Depending on where you will be placed, there is a supermarket in walking distance. You can also get beer, disposable BBQs and normal food. And it's not a problem to bring that stuff onto the camping. It is however prohibited to bring glass on any site of Wacken, camping and festival site! On the FESTIVAL SITE, you are at the mercy of the vendors over there.
I cannot answer to the rest of your questions. Hopefully somebody else who's been flying to Wacken, can answer them.
Just a quick reminder: a friend of mine took 3 bags from Baltimore MD, and had pay a fee of $50/€45 per flight! Try to limit it to 2.