Your PhD in language history has misinformed you. First of all, the "Japanese word for king" is a Chinese character, so the etymology would be Chinese, not Japanese. Here it is: 王, pronounced おう (long "o" sound, continued for a beat) in Japanese and wáng in Chinese, where it is also a common family name.
Secondly, your etymology is faulty. The character is close to 3,000 years old. Your complicated etymology of "The mediator between Heaven and Earth" is mistaken. No one really knows the etymology, but it certainly is not that complicated.