The thing is, Shiro isn't the last descendant of the king of Okinawa. He's a descendant of the last king of Okinawa. One of, presumably, many. Michael, being the last descendant of Charlemagne is incredibly unlikely, because effectively everyone of European heritage is descended from him, the dude had 60 great grand kids. Royal families don't just have one or two kids, generally.
And, like the last king of Okinawa, many of those royal titles have ceased to be. Even if you can prove your direct lineage, you show up to Normandy saying you're the Nth descendant of Robert II, they'll say that's very nice, and then arrest you if you try to take over the government.
Maybe it's true that the Knights are all in direct line for whichever throne. Possibly. But that would mean that Michael, who happens to live in Chicago, AND Murphy AND Susan AND Butters are all secretly the One True Heir. That there are no other better claimants to four different thrones, and also they all happen to be buddies with Harry. Oh, and Harry himself. And that's only if he gets it through Malcolm, because otherwise Thomas would be the one who gets it, being older.