> How do you determine to whom an IP is even registered to?

You check the RIR's records.

> They get sub-leased all the time.

With records updated. If not, any consequences from wrong information fall on the lessor and lessee.

> There's also no way to actually know _where_ an IP actually originates from. Only its AS path.

Ping time from different locations on their upstream AS gives a good guess.

> With records updated. If not, any consequences from wrong information fall on the lessor and lessee.

Not always + there are no consequences whatsoever.

Plenty of leasing services will just provide you with IRR & RPKI, without ever touching the actual records.

> Ping time from different locations on their upstream AS gives a good guess.

Upstream AS is meaningless if it's a T1 carrier. Ping AS6939. They are everywhere.