Taboos... Well, starting a discussion on immigration or a few other political subjects might not be the best idea, and calling the country "Holland" won't make you popular in certain parts. People who actually live on Holland probably won't care, but don't do it in Frisia for example.

Perfect English? Absolutely not. Expect horrible accents and the occasional vangaalism (named after Louis van Gaal). Will they be able to make themselves understood, and understand you? Yes, you can expect them to. I believe about 90% of the population speaks English. That's not that much less than the US (something close to 94%). Moet people that don't are probably older people.

Don't know about concerts being violent. But I don't know much about concerts.

Gas stations do sell diesel fuel. As far as I can tell, it's the same thing. Don't quote me on it, though.

I don't think that singing songs at the camping is a very Dutch thing, although it may happen if there are a lot of teens/young adults.

Dutch people probably drink mostly beer. Beer is definitely insanely popular. I'm not sure about the popularity of mixes or liquors (although I expect mixes to be somewhat popular as well), but you're definitely good with beer. It's usually cheap pilsner. Things like Grolsch or Heineken, depending on what's popular in the area. I wouldn't expect any really good beer. Unless you like those big pilsner brands of course. A lot of people seem to. A lot of people don't even know that something other than pils even exists, though.

I think fake politeness might be offensive to the Dutch. Don't try too hard to be polite, be genuine. Don't ask how someone is doing unless you genuinely want to know and genuinely care. And if they aren't doing well and you ask, they may certainly just tell you. The Dutch are a genuine people. That's part of the famed Dutch bluntness. Basically, just be yourself. The Dutch usually value people who are just themselves, at least, as long as they don't act too different or weird. Although at festivals, you can of course expect a lot of weird. I don't think you can act too different or weird at those. And don't brag. Bragging about lifestyle or income absolutely will not impress them. They may ask about lifestyle if they're curious, but genuineness is key.