To address all of your points in order:

1. There is no reason to believe that this must be accomplished via coercion. Voluntary cooperation can accomplish an organized defense against an external attacker. This has been the case in Rojava, where the Kurds have experienced a fair amount of success repelling ISIS and taking back territory. moreover, a decentralized resistance is much more effective in demoralizing and bankrupting an enemy. War is costly. Guerilla warfare is much cheaper and can be just as effective. For example, Afghanistan in the 80's and today.
2. That's why voluntaryism will function when a sufficient number of people subscribe to the universal ethical principles of consent and self-ownership. Anarchists that think that anything constructive in the long-term by violent revolution are fooling themselves. Politics is a lagging indicator of culture; when there is a sufficient culture shift, nation-states will fall by the way side. Your example of water rights presupposes states existing, so it's not indicative of whether or not an voluntaryist society could function.
3. Says who? Assertions aren't evidence. In a voluntary society, especially one with more technological advancement and access to information than ours, reputation would be vitally important. If you owe someone money, and you refuse to handle your business in a legitimate manner to reach a mutual agreement, you can suffer serious financial and social sanctions simply by people refusing to do business with you or interact with you. Once again, the power of ostracizing and shunning people is severely downplayed.

I'd also like to touch on one other thing: the fact it has not succeeded before does not mean that it cannot succeed in the future. If you were to tell someone in 1789 that they would be able to communicate on devices that harnessed electricity to send messages and voices across the globe instantaneously, and that these devices fit in one's hand and could also store information, view pictures, and view a moving series of pictures, you'd be ridiculed into obscurity. And yet, here we are.