i think you all are reading too much into this, if a second korean war happens, it'll still be the US who are gonna be the main force against north korea, japan plays a supporting role at best.
as for japan and its war criminals/apologies etc. the problem isn't that they did not show remorse or apologize, because japan has made many statement of apology and remorse. the problem is that right after they do they then immediately undermine their own apologies by doing things like questioning whether massacres or force prostitution happened, the fact that these debates and questions are coming from politicians in power, spits in the face of the millions that they killed. meanwhile in germany, the swastika is banned and no public leader would ever question whether the holocaust was real., see the difference?
as for the south koreans, this doesnt mean they are some how "under the protection" of japan, just that there is another ally (implying equal status)in case sh1t hits the fan, they know the only one they are "under the protection"(implying hierarchical status) of is the US