Check out https://matrix.itasoftware.com/ for flights. It's what I use whenever I research airfare. It will give you almost every conceivable route and airline to where you want to go. It's a pretty powerful tool. Only caveat is you cannot book through the site, you'll have to go through the airline. If you find a better price on Expedia or something call the airline and see if they'll price match. Always better to book directly with airline over third party.
1) Depends on your definition of cold, I suppose. The weather forecast usually lists a "minimum temperature", and that tends to be accurate and this summer is looking good. https://weather.com/en-GB/weather/tenday/l/91ad8b00cbd1c0bb253262a49a6ee13a52b45a523098a7ce5693d92434f07157 lists the forecast for the coming 10 days. If you want to be safe, pack some extra clothes to maybe sleep in and/or get a cheapish extra blanket when you're heading out from Hamburg, and you should be fine. Ikea in Schnelsen (...district of Hamburg) is close to the road to wacken anyhow, so it's not much of a detour if you feel the need to stock up on cheap blankets. The way it's looking atm, you'll probably need ....
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2) ... Rubber boots a lot more. They are an absolute must. Wacken is legendary for the muddy conditions and I've seen some crazy stuff over the years (watery-mud knee-high? yes, sir). You can usually get them at the site / in town at reasonable-ish prices though, if you're in a bind for packing space.
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3) every camp ground apart from 1 or 2 are car-park. you'll camp next to your car, so no need to lug your stuff around the site. There will be traffic guidance by the festival wardens. Just turn off your navigation (satnav/app) once you see the first signs directing you and you'll be absolutely fine. The level or organisation and structure is also something Wacken is legendary for :).
I have used these for a lot of festivals and indoor concerts. https://www.amazon.com/Etymotic-Fidelity-Earplugs-ETY-Plugs-Standard/dp/B0044DEESS
They come with string that you can attach to your earplugs. This way you don't lose them.
Accuweather shows good weather for the Festival days... https://www.accuweather.com/en/de/wacken/25596/daily-weather-forecast/181290
I would suggest using Google Flights to search. You can compare airlines and days, and also have it notify you by email when prices change for routes you want to fly. I personally fly United from Houston to either Frankfurt or Amsterdam then connect via flight to Hamburg or drive every year. If you book the connecting flight on the same reservation, the connecting flight will likely be on Lufthansa.
I use https://matrix.itasoftware.com.
I'd say right now prices are probably at their lowest or won't go that much farther down. I've already seen them going back up out of MSP. Playing the low fare game can end up biting you in the end because trying to time airfare isn't an exact science. It is said airfare is "usually" at it's lowest 9 months out for international travel.
Are you sure you have no room to bring your own tent? Amazon has some pretty nice tents that come in very small packaging. This one for example. It only weighs 4 pounds and you should be able to either strap it to you backpack or fit it inside. That's what I've done the last 2 trips to Wacken.