Answer the Questions on the Scenario That Might Happen on One of the Islands between Japan and Korea.

Example:
North Korea bombs the island (coordinate: 37.239135, 131.867994). Because of this, dozens of South Korean (ROK) civilians are injured or killed, and facilities built and occupied by the Republic of Korea are destroyed. ROK requests the USA to retaliate on North Korea based on “Mutual Defense Treaty between the Republic of Korea and the United States of America”.
Questions:
1. If U.S.A decides to counterattack on N.K with one of its allies or alone, will its legal basis be “The ROK-U.S. Mutual Defense Agreement” signed in 1953 or one of the other treaties that the Republic of Korea did not sign?
2. If “The ROK-U.S. Mutual Defense Agreement” is useless in this case (example), explain why.
3. Explain whether “The U.S.A-Japan Security Treaty” can be used or not in the case.