Interestingly, G. japonica was long considered a subspecies of G. spengleri; this is where the literature confusion would have originated.
You are lucky to be present on such a beautiful island as Okinawa, as it is not just herps that have some interesting endemics (go find a very rare, primitive rabbit). The Cuora flavomarginata subspecific designation for animals on Okinawa has been contested on behalf of the description relying on arguable characteristics. Outside of a few scute measurements, they're pretty much the same as the ones on the Chinese mainland.
Indeed, G. japonica is protected in full, so as the other post already read, hands off. Should you ever live (again?) in the U.S., perhaps you could splurge a couple thousand to buy one or two, heh. If indeed you find this species or any other chelonian of interest, it would be most wonderful to post some pictures on here, as in-the-wild photos of most lower archipelago species are lacking outside of Japanese publications. Thanks.