On the other hand, the KC-46 starts as a new FAA-approved 767-2c, modified by STC with installation of numerous military subsystems and finally provisioned with certain military systems and functions that meet only military airworthiness requirements.
True, but the 767-2C is built with "provisions" to ease the transition to the KC-46 without having to take everything apart - for that reason the 767 line is now ITAR (even though they're still building 767-300F on it). They don't add most of the military stuff during the assembly - they fly the "737" to Boeing field, push it across the street to the Thompson building, then turn it into a P-8. Similarly, they build a 767-2C, push it to the other end of Paine Field to the Everett Mod Center, and turn it into a KC-46.
In concept, the 767-2C/KC-46 is very similar to the P-8. In practice the KC-46 still has a ways to go...