Can U.S. mediate between Japan, S Korea to ensure regional stability?

The only way the US can mediate is to reveal that the radar signal that the P-1 detected was the X-Band search radar from the Korean coast guard ship(The analyzed signal from Japanese released audio clearly revealed it was from a rotating radar with periodic peaks, not from a tracking radar like the STIR-180 lock-on radar as JMSDF has claimed) and ask JMSDF to apologize to Korea and upgrade P-1's ESM equipment because it is not upto the Western standard.

https://en.wikipedia.org/wiki/2018_Japan%E2%80%93South_Korea_radar_lock-on_dispute#2019

There is no way the US can mediate between two parties where both come out winners or at least even.

The South Korean need to abide by their agreements. It is really that simple

A rogue state like the US is incapable of mediating anything. Why would any nation accept mediation by a corrupt, warmongering administration.