How do we determine that you assaulted me? It's your word against mine. We're not talking about the complexity of determining the true account of events, which is obviously great. We're talking about the complexity of establishing what property rights are. It's not complicated at all.
>What about if I wrote a contract to lend you my laptop and we didn't specify a certain edge case?
Then common law, which springs from the first principles of personal and property rights, would preside.
We'd have a similarly difficult time determining if a hockey fight classifies as assault. Or if in a traffic altercation, you stepping out of your car and aggressively approaching me justified my preemptive (or aggressive?) first punch.
>There are answers to all these questions, but we need a complicated system of laws to determine them.
The system of laws being complicated does not mean that it is not generally straightforward to determine what a person has property rights to. The complexity comes from the edge cases, but most cases are not edge cases.