In a cooperation, you're expected to give money without having a say on where it goes, isn't it? You only knows that it benefits all the members collectively and you give money because of that very fact. Some cooperation expects you to pay once and they use that money for investments. Some other type of cooperation expects you to pay annually and this money is used to help members without having to ask for money every time. The cooperation can buy wheat flour to prepare for famine, but you don't eat bread, so you have no need for flour but the others do.
As for insurance, well, you're using a very detailed insurance there. With insurance, you pay for the general stuff. Let's take the average health insurance. You get hospital benefits, medicine benefits and when you die, your relatives get some money. You pay every year, but do you use it every year? Probably not, but your money that you don't use helps pay for people who do need the healthcare, and on some years that you do need the healthcare, you use some other people's money to pay for your bills. For example, you only paid like USD100 per month, but one day, you got into an accident and in that month, your healthcare bill is USD10000, now where would that money come from?
Or, let's go with your example, auto insurance. Now, this is a very specific insurance, concerning itself with only one thing: automobile. Now, not everyone has a car, and taking a car insurance without a car is pointless, and even stupid in hindsight. So the people taking the auto-insurance are people who have a car. If you have a car, you take the auto-insurance, then when you do get into an accident, you use other people's money to fix your car. You take an insurance out of fear that something can and will go wrong, though in some countries it's a law to have auto-insurance if you have a car.
You give this money to the community because you are selfish. This isn't charity. You want an insurance, in many different ways, because you are afraid that something bad will happen to you. You and neighbour give your money to this community, because if either one of you got in trouble, you have to help each other, but neither of you wants to bend over backwards to help each other with everything you have. You expect the whole community to pitch in, but your community probably doesn't want to pitch in. There's where community funds (cooperation) comes in.
A government (and taxation) is essentially the same thing. Sure, it's not voluntary and I will not argue with you about that. However, they perform the same function. You give money to the government, and the government either make things that benefit you or help you when you're down. You don't have to bend over backwards to help your neighbour, the government takes part in helping your neighbour.
You want to go to university? The government can give you a loan (where you pay for it after you graduate), subsidize it (so that it becomes cheaper for you) or give you a scholarship (where you don't have to pay a single cent), with everyone's money. Each person giving maybe $5 for your education. Now imagine asking 4,000 people $5 to get $20,000 to fund your education. Can you do it? Let us ignore the fact that you probably have the money to pay it yourself, and let us assume that you're a 17 year old kid with no asset or rich parents.
Let's ignore American politics, Rothchild and the IRS, because your new seasteading community will likely not have their hand in it. Someone will probably mention or think about it the moment taxation comes up in American context.
Exactly, nobody really understands what it is. Everyone seems to have a different understanding when the word libertarianism comes up. If you ask 100 people what is libertarianism, 100 people will give 100 different answers. Even in the same group of libertarians, their answer will be very different. The only thing that everyone will agree on is that libertarianism makes emphasis on personal freedom. Some would not even answer beyond 'no taxes, do whatever you want, no government' and then point you to the wikipedia article because the wikipedia article explains it better.
Marinea's brand of libertarianism (not that I really know in depth as I'm not a member of Marinea) is likely different from Minerva Project, Project Entropy or the Blue Sea Project's libertarian ideals. For example, the Blue Sea Project do not care about enforcing self-ownership, as they're based on the original Utopian ideal. That is, every citizen can do whatever they want, can take whatever they want, and live wherever they want. They won't even have to work, because they have slaves to do that for them. Yes, the original Utopia promoted slavery.
There are so many different brands of libertarianism, some with conflicting ideals and most with vague ideals that only your own brand of libertarianism matters. This makes it a problem, because an argument about one project will most likely be a non issue with another project, Arguing for or against libertarianism is hard because the answers are different for different projects but they're all called libertarianism. So when you argue about something, you're bound to offend someone else because their brand makes that issue a non-issue. Arguing about communism is much easier in comparison, because you have Marxism, Leninism, Stalinism and Maoism. They're all communism and they're all the same, but there are subtle differences that sets them apart, the same can't be said about libertarianism.