I wouldn't say entirely irrelevant. Rana Mitter in The Forgotten Ally makes a compelling case, for example, that the fact that a good part of the IJA was stuck in the unwinnable Chinese quagmire was important to the island campaigns.
But anyways: if Sky puts in a Japanese admiral, it's only because he intends on making that poor fellow suffer because he is literally stuck in the worst possible situation ever. I don't speak for Sly but I think it would be, uh, thematically off. Every other admiral would be trying to do something to save their girls and salvage something from their nation; the Japanese admiral would be getting drunk off his ass as he realises that everything he does either prolongs the war, getting a third city nuked, or gets him assassinated.
The IJA is worthless in the Pacific War outside of the critical islands that the USN/USMC needed to capture in order to advance on the Japanese home islands. Any IJA forces garrisoned at other islands are literally a DRAIN on Japan's war-fighting capabilities. There's a reason it was called 'island hopping', in that the USN/USMC hopped and skipped right past several garrisoned islands and only hit the truly important ones. Once they had those, any islands further out were literally starved into either submission and surrender, forced into abandonment, or, in the case of a few of the smaller ones, quite literally starved to death to the last man. Until that happened, several of those larger, bypassed, islands still HAD to be supplied, and thus any convoys to them made for incredibly easy targets for the USN's submarine forces which went through Japan's merchant fleet like a thresher does to a wheat field.
The IJA could have pulled completely out of China in 1940, moving every single unit they had into the Philippines and everything north of the Philippines all the way up to Okinawa, and it still wouldn't have done a damn bit of good outside of making Iwo Jima even bloodier, and slowing, but not denying, the fall of Okinawa. Hell, it would have made things WORSE for Japan as there would have been even more troops per island needing constant supply runs, and thus, even more easy-to-kill convoys for the various USN sub packs to kill.
The best bet for a Japanese Kanmusu admiral, is to get onboard one of the flagships (Yamato, Kaga, or Ooyodo for example, as a short list), get a hold of its spirit, and WARN IT of what is coming. Not just with what will happen in the war, but that the Abyssals are coming. Warn it, and do his best to make the spirit transmit as much of that knowledge as possible, as widely as possible, to the rest of the spirits. That and flat out telling them what will happen during the war, that there's simply no way for the IJN to stop America from burying Japan's navy under sheer numbers, yet not to grieve too much, because they will return later to fight a truly worthy enemy - one that fights against all of humanity. Hit the samurai tones right, and they'll be eating out of his hand. Sacrifice everything now, even fighting an opponent that shouldn't be, in order to be ready for the upcoming REAL war. After all, the first few months/years of the Abyssal war had to have been chaos unleashed in dealing with their seemingly insane tactics/strategies, how to counter them, and then how to use the returned Kanmusu properly, let alone supply them. Drill it into the IJN's ship-spirits, while still spirits, about all that he can remember from the Abyssal war, would allow them to possibly perform miracles almost immediately after being summoned due to knowing what is needed to do, and not to do, rather then chaotic on-the-job-while-under-fire training/experience.
Then?
Then, the officer in question
does his duty to the best of his ability, and either dies fighting the USN using all the tactical tricks he knows from future knowledge, or miraculously survives the war. Very likely will end up being one of those 'father to his men' types that personnel would gladly/happily march into hell behind, while also having a virtually spotless record for POW treatment/picking up survivors. That is, if the aforementioned 'dying in the war' doesn't happen. But he is still duty bound to defend Japan. Working out of step would mean that he might be allowing one of the xenophobic 'kill the survivors in their lifejackets/gun down the lifeboats' war-hawk to instead fill his shoes/position, rather than him working subtly at the few things he CAN change.
-=-=-
As I mentioned previously, NOTHING that Admiral can do, will prevent the war from ending on Japan's footsteps, and certainly not without at least one A-bomb being dropped. Well, outside of a full-up miracle where Yamato herself materializes physically before Yamamato and/or (possibly more this) the Emperor himself, and explains things. Outside of that, all he can do is cause the war to be prolonged, but still, never won. And again, if its prolonged too much, the ending could be far FAR worse. Prolonging the war by as much as 6 months, delaying Operation Downfall (both Olympic and Coronet), might very well end up seeing A-bombs being dropped ahead of the beachhead landings, which would have badly damaged parts of Kyushu, while utterly decimating the Kanto Plain around Tokyo (Hiroshima? Nagasaki? Their death tolls would have
PALED in comparison). And that's prior to the actual invasion landings, let alone the slog into the mainland that was expected to occur.
Thus any Kanmusu time-traveled officer has to swallow his dignity and pride, and act on a much MUCH longer time frame than anyone else - because no matter what, he's doomed. Try to stop the war? He's dead/replaced. Try to prolong it? His nation is even more of a corpse than in the original timeline. So the only thing left is to alert the ship-spirits, and get them ready to endure the Pacific War, while readying them for the Abyssal War.
