Wow, you really want me to expound on an entire philosophical system in a few short paragraphs?

I'm not trying to cop out of this discussion, it's just that these are all things that have been covered numerous times. There are a lot of very smart people spending a great deal of time enumerating all the complications you seem to think are deal-breakers.

I'll address you more generally, then, and pick a few big ones to respond to.

Anarcho capitalism, and the two rules I mentioned. They don't just encompass a legal system, but a philosophical system as well (like you mentioned). The idea being that if we can come up with basic set of rules that are moral, and we don't break them, then we have come up with a moral/fair judicial system that is grounded in fairness for all because the same rules apply to everyone.

"What's a valid contract?"
Anything that you voluntarily enter into and whoever you've chosen to help you arbitrate disputes with the other contractees. You're free to do so, even if you enter into a contract to do bad things. The contract is valid between you two, but you will no-doubt get in to trouble for the things you end up doing. Again, a big implementation-specific question. No morals here. But if you must know: most arbitration committees/courts/whatever you want to call them will probably not look kindly on contracts that break the basic tenets of harm/property.

"What about harm caused to the polity by non-aggressive use of force (drunk driving without regard to other's lives?"
First of all, in a free society, there is no "polity". Second, reckless driving is handled by private property. Whoever owns the road you drive on sets the rules; if you don't abide by them, you can't drive on that road because it's private property. It's already that way, really, it's just that you don't notice it because you see the state. If someone wishes to create a set of roads where people can drive recklessly, then others are free to use other roads that agree to their sense/level of security.

"What happens when someone breaks your stuff and can't pay for it?"
Insurance, on either side. Look past the state, and you will see ingenious solutions to remedy and dampen risk/loss.

I'm on my third one, and it seems you're just trying to come up with contrived/weird scenarios. Solutions to these problems are just endless, but remember, they must not behave in an immoral way (by breaking the two basic rules mentioned in my previous post). Perhaps if you have a taste, you might be curious to read up some more.

"There is no bright line, every issue has to be considered separately and people will always disagree."
People disagree now. It's just that you have no choice in it at all. We're all forced to "agree" on a certain set of arbitrarily and "consistently" chosen set of rules that only a majority of us supposedly agree on.

On a side note, I look forward to the day when free-minded individuals can come up with solutions to all these problems in a peaceful and non-coercive manner. Until then, we'll be stuck coming up with utilitarian solutions.

If you're curious, I can point you to some resources/books regarding anarcho-capitalism and related topics. There are some nice introductions out there; some of which detail all the possible combinations of problems you can think of.