Korean, Chinese, and Japanese historical artifacts confirm a Japanese presence on the Korean peninsula where Mimana would have been.

Can you link to some reference to this? As far as I'm aware, there's been no actual evidence of a Japanese state on the Korean Peninsula dating to that period, which means that basically everything is speculation without evidence behind it.

There is no real evidence for it. See page 27 to 29 of The History of Korea by Chun-gil Kim - it's a short book, but a good introduction - but to be brief, there is no archaeological evidence supporting the existence of a Japanese "colony" or even a proper state in the region.

There might be a Japanese presence, yes, considering that both the Gaya confederacy and the Baekje kingdom had close ties to the archipelago. We know that the Japanese were allies to both for centuries - the Japanese fought the Goguryeo army, along with Baekje and Gaya, when it attacked in the late 4th century. Even when Baekje's king had surrendered to the Tang-Silla coalition and the kingdom had already collapsed, the Japanese sent forces to aid Baekje rebels against the conquerors. There have been some theories that Mimana may actually have been a Japanese trading or diplomatic outpost in the territory of Gaya.

Yet there is no fundamental shift in Gaya archaeological sites, as might be expected if Japan had conquered the region somehow. And how would Japan even have managed? The archipelago was hardly properly unified during the time that "Mimana" would have been established, while the lower Nakdong Valley of the time did loosely recognize the authority of the polity centered around what is now Gimhae.

EDIT: I should mention that there are theories that Gaya was Japonic-speaking, but speaking a Japonic language does not mean they were Japanese, any more than Danes are Germans.

Can you elaborate on what it actually is?