... as neither of us are emperors. In a nation of ~ 360 million people, we are just two men who enjoy creating content on our Steem blockchain, from time to time. Likely no different than you ...

The President of the United States was elected among Protestants and Catholics. The President of the United States is a citizen in the United States, but is perceived by South Koreans, Chinese, and Japanese as emperors of the American Empire.

Interesting to read your perspective of the pressures Korea feels from China and Japan. From a great distance, what I read of the Korean peninsula, the North Koreans are very closely aligned with China, on whom they depend for survival.

You are right. However, China seduces South Korea by offering enormous economic benefits and territory. In particular, China seduces that if the USFK withdraws from South Korea, China will also give Korea unification.
At the same time, China threatens to have 100 nuclear missiles aimed at South Korea. China said the United States could not block all 100 nuclear missiles.

The South Koreans, economically speaking, have more reason to be allied with Japan. As much in their own self-interest, it would seem, than anything else. As both parties have reasons to be concerned with the threat posed by North Korea ...

Your words are theoretically correct. However, Koreans, especially Christians like me, do not want to be ruled by the pagan state of Japan.
Even the great Protestant Americans @roleerob did not make Japan a Christian nation.

In terms of Korean history, sentiment, diplomacy, military, and economics, the alliance between Korea and Japan is less than profit. In particular, China desperately opposes the Japanese army presence in Korea. It will be angry with China.

I believe that while Korea has a military alliance with the United States, China and Japan must have economic, cultural and diplomatic relations.