I also don’t understand how mentioning JSON is even relevant in this thread.
This sort of dispute is probably going to end up in a legal forum at some point. In that case, video footage from a device that's not physically on the bike (and therefore much harder to tamper with) is going to carry much, much, much more weight than GPS traces. Also, you usually know exactly who is supposed to have access to CCTV camera systems / footage, and can therefore have those people make legally binding representations as to their accuracy / fidelity.
(Of course, with regards to this problem, your objective as a service provider is to end up in court as infrequently as possible. You'd be more likely to invest in anti-theft measures so you can get the probability of theft low enough to reasonably write off as part of opex.)
Combine this with a few anti-theft procedures that don't involve GPS / internet (as commonly used by bikeshare programs in various municipalities) and you've got something that might work.