If a Japanese civil war happens (and I'm quite sure pdf has used red herrings before), the great powers' attitude will be driven not only by such cold calculations, but also by their representations of what is happening and by their capabilities.
First, I am decidedly unsure they will draw such a sharp line between the IJA and the IJN. We on here have decided that the IJA were the crazy ones and the IJN the reasonnable ones, but this is largely because we have Word of God on this, which allow us to see through their calculations and cast them in those roles. But from the GPs' standpoint when the first shot get fired, it wont be so easy. The situation, at least in the beginning, will be massively confusing - it's not like Yamamoto will send a cable to Washington saying "look it up on Althistory.com", opening direct conducts of communication will take a while, getting reliable info even more so. The actors will be stuck on their preexisting representations of the IJN and the IJA. For the average American, it will probably just be "a bunch of Japs shooting each others". For Stalin, it will be a trap and he will probably lose three months trying to get ahead of a non-existent capitalist plot before deciding on a course of action (like he did in Spain). The idea of helping the IJN will sound about as reasonable as funding a retirement home for man-eating sharks to most westerners. There might even be significant dissensions within the Entente - for Britain, the Far East is vastly more important than it is for France.
Second, we have to keep in mind that the GP can do, well, things, but not anything. For instance, a Soviet direct intervention is ASB until the IJN get severly mauled; the Entente has the sea power to bear, but not the land power that far away from Europe; the US has in theory the most hard power on theater, but it will be hard, hard, to sell a direct intervention to the public (until the IJA decides to do their own Pearl Harbour. I can definitely a "day of infamy" speech after someone in the Kwantung army has 70 American missionaries in China hanged.). So they each have a range of options, from economic sanctions to all-in, but their willingness to resort to the most complicated and costly options will be depending a lot on their assessment of the situation. For instance, a british direct intervention is highly unlikely in the beginning, but if the IJA occupies HK it becomes a no-brainer at least until HK is secured.
In all, I think a lot of the actors' attitude towards the belligerents will be shaped by a relatively small number of events in a very short time. If nothing happened and the Japanese just shot each other quietly for a few months, à la phoney war, then sure, the GP would quietly make their calculations; but it is much more likely imho that there will be "incidents" - big ones. What happens if foreign nationals in Japan are harmed during the fighting? If the IJN manages to get the emperor to do their PR? If the IJA in China walks into HK, the concessions or KCW? If some idiots decide that dragging in the US is the best idea to defeat the IJN or force them to unite in a foreign war? In all, a lot of things can happen that would either confirm the GP actors' preconceptions or allow them to frame either side as the baddies and spare them the trouble of trying to figure it out through the FOW. For the public, and to some extent for deciders, they will determine which options are "feasible", "necessary" and "out of the question". So, instead of trying to calculate objectively which outcome of the war would suit the GP best, we have to try and figure out what the Japanese factions will do and how it will shape the GP's fluid assessment of the situation.