I think we are over-complicating things here. Let me try to simplify. If you buy a one way ticket JFK-NRT-TPE and don't check any bags, it is highly unlikely anyone from the airline will do anything if you get off the plane at NRT and don't use the last leg of the ticket.

You have a big problem if it's a round trip ticket, but not on a one way.

Whether you will have immigration problems at NRT is a separate problem, but if you dress nicely and act well, I'd doubt it. Bear in mind, however, that air tickets to the rest of Asia from Japan are very expensive. When you see the price of the NRT-TPE flight bought inside Japan you may be shocked!