I don't think I am naive, and I don't think you are cynical, either. You are skeptical.

Federalism worked pretty well in the past and it can serve us again. It isn't a pancea. Some issues like slavery were simply too much to take, even if you knew it was happening in a far away place. And slavery, among some other irreconcilable differences broke federalism and led to the civil war.

But after the Civil War federalism continued to operate with a lot more strength than it does today, just with some adjustments like...no slavery and no leaving the Union.

Everybody have issues that make them unhappy, and it bothers some people more than it bothers others. I'm certainly not going to engage in civil war over abortion rights, either for or against, but for some people, that's an issue worth fighting for.

Our genius is our ability to compromise, even if we would deny it. The trick to peace is in providing enough satisfaction in each state so that people feelTHEIR state has its head screwed on straight and the know that while other states are terrible it's not worth fighting over. As long as we can prevent a critical mass of people from boiling over with the desire to attack their countrymen, we keep the peace.

So to answer your points:

People get their way in their states. Democracy. For example, California may decide to legalize cocaine, but Texas will give you five years for it. As a Californian, you figure that is terrible, but it's not your business what goes on in Texas, and at least California isn't is a crazy redneck police state.

That's an example of federalism. Yes it is possible cocaine will travel from California to Texas which creates some friction between the states, but Texas will lock you up if you bring it in, and California gets to keep its freedom. Everyone is satisfied.

Does Minnesota get to dump toxic waste in the river and send the problem downstream to Mississipi? No. That's a federal matter because it is an interstate dispute. This sort of law is well-sorted by now.

More seriously, can a state which disallows easy access to guns do anything about other states which allow it? Not really. All they can do is punish harshly anyone caught breaking the law in their state. And moreover they may be limited by the Bill of Rights and supreme court interpretation.

There will always be some disputes between the states. The question for you as an individual is whether you find the fact that the laws in some states are SO intolerable, like slavery, that you would go to war against your own people to force them to do as you believe they should?

Or could you find comfort and solace in knowing that your own state is a bastion of sanity and put aside at least enough of your anger not to take up arms?

It is said that the purpose of liberalism (in the classical sense) is peace. So too is the purpose of federalism.