Pretty sure if they live by themself and nobody else comes into their dwelling and there is no other name attachef to their subscriber info it does

Nope... wardriving? Spoofing? Too much uncertainty to convict with. Basis for a warrant on the property? Yes, probably.

What if they are on cellular and that hasn't been upgraded to IPv6?

Years ago I handled fraud cases for an e-commerce site with local police, at some point they started asking for IP and port numbers for the offenders, rather than just the IP. Turns out that one of the cellular phone providers had basically run out of IPv4 addresses for their 4G network and did some NAT solution. If you didn't have the port number the client had connected from then they could only tell you which cell tower had been used, not who the customer was.

Do you know if there's a guide about all the ipv4/6 stuff and optimal internet settings for Mac or more high-level generally?

Definitely not. I still am logged into my ex girlfriend wifi so if I wanted to harm her I could easily go stand outside her home at night and download malicious files. That would not make her guilty. They may investigate but that is not proof she did something unlawful.

That's remarkably—err, trusting—of her to not like change her WiFi password after finishing up with you. Yikes

Case law in what country? Mullvad is Swedish, are you knowledgeable about Swedish case law?