> Will everyone really always have what they need, or is that a theoretical state that is actually very unlikely to be achieved?

Just like our current system, there is no guarantee that everyone will have all their needs met. What differs, though, is what happens when those needs aren't met. On one side all you can do is complain and hope, on the other you get to raise the point to the community and you share the power to solve the problem.

> What happens if more than just one farmer opts out of the anarchistic lifestyle, they embrace capitalism, and outcompete the anarchists?

An anarchist society will produce enough for everybody. You can't outcompete what is already available for free through the community.

> Could that ever reach a point where it would be necessary to stop them by force in order to preserve the anarchistic society?

If that's what the society decides, then yes. It's all about the community decides for themselves.

> and not themselves in particular (which I think is more realistic, on average).

That's actually not true, the myth of the individualistic human, sold with stories of the proud family man who is the only one on earth who's able to save their progeniture, is largely wrong.