2) With no Washington Treaty does Britain retain the 10 Year rule after the Chanak Crisis with Turkey. This toggle gives a plausible reason to accelerate British RN and RAF build-ups.
Interesting point about the 10 Year Rule and Chanak, both of which I've only become aware of through reading up on the subject. Personally I think the lack of a Washington Treaty changes the game far more than an essentially ephemeral crisis in a decaying power.
Among the things I find interesting about Chanak was that this emphasized the hollowness of the Allied victory after the Great War. The prize was the Bosporus, a premier chokepoint since Homeric times, and by far the weakest of pre-war Great Powers, the newly emasculated Turkey, had forced Britain to withdraw from this key location without a fight. An aggressive Japan, in our Washington Treaty-free world can only look at this as a sign that her own aggression won't be strongly resisted on the other side of the world. The British almost have to revoke the 10 Year unless they tie up Japan with a non-aggression pact. If they don't do either, that points back to a "head in the sand" posture. And you're right that people "usually get there in the end", the question is too often how many lives are lost before the stakes are clear because the gold standard or unemployment or the "Irish Question" or Commonwealth organization or another uprising on the Northwest Frontier crowded out the government's attention in the meantime.
does Japan really want war with the US, or just to cover its flanks while it makes a grab for the oil in the Dutch East Indies? A thrust south leaves the Phillipines a threat on the sea lines between Indonesia and Japan, but also puts the British and French on their other flank in Malaya and Indochina. Britain's traditional affection for the underdog will put any alliance under immediate strain if Japan attacks the Dutch. And if the British are a threat, then Hong Kong is a knife at Japan's throat. By an accident of geography, the one thing Japan wants most is held by the weakest colonial power, but is located such that she has to contemplate war with all if she wants war with one.
These are good points. A one on one war scenario might be a scenario like this: No Washington, No Nine Power Treaties, an Anglo-Japanese non-aggression pact. Japan secures long term contracts to DEI oil. Japanese ambitions in China are hostage to US bases in the Philippines that threaten the DEI oil so the US threat needs to be eliminated, ala Russia's competitive threat to North Asia before the Great War. In game terms, you would almost have to make wholesale base modifications to take the Empire out of the game and just give the Japanese defacto oil and supply heads in the DEI with a house rule that the US couldn't attack the bases. Outright conquest of the Indies is postponed until after US pretensions of hegemony are addressed. Alternatively, one could increase oil supply in Formosa on the assumption that the Royal Navy escorts British and Dutch flagged merchants to Japanese ports. In any case, Japan would be accomplishing a diplomatic tour de force by isolating the USA from potential military allies before the outbreak of war. Britain would eventually have the opportunity to use her good offices to broker a peace as TR did between Japan and Russia at Portsmouth (and win the Nobel Peace Prize).
This is a big speculative stretch, no doubt about that. However, from a play perspective, this gives a more balanced scenario that doesn't tilt decisively against the Japanese until deeper into the game when US industrial output becomes fully engaged in the war.
Japan doesn't seem to have had the diplomatic subtlety to be able to sell that view widely in WWII, I don't see them doing any better in the 20s.
I'll gladly plead guilty to recently finishing John Toland's The Rising Sun where he spends quite a bit of time presenting evidence for aspects of this pan-Asian idealism motivating some advocates of Japanese aggression and being captured by considerably baser motives on the part of others in the Japanese government. I think it was Gilbert's history of the 20th century that cites wide spread anti-western riots in China in 1927. This might be very much a situation where the Chinese actors would view Japan as the lesser of two evils. There is a game question here that can be addressed as well. This sort of mod give the Japanese enough manpower to conquer China and head for India. Perhaps for balance sake, Allied ground reinforcements will need to increased somewhat for the British Empire, but it makes the continent a major theater of operations. Maybe that means that Japanese air power and ground forces get bulked up a bit at the expense of some of those hypothetical battleships. If the ground warfare is a question you want the a scenario to address, then this scenario takes you there.
As another alternative, maybe there is a 20th Century Sepoy Revolt in Burma, Indochina, or the DEI. The Japanese recognize the new regime so that leads to war between the Europeans and Japan. In this war, the US stays neutral, mindful of its own ex-colonial status. The US suspends oil shipments by way of neutrality so that locks Japan and the Europeans in a fight for the DEI. That may offer the opportunity to float the French Navy east of Suez.
The most important nugget when it comes to the scenario assumptions is the question of what sort of scenario are you really aiming for: various rearmament races leading up war, a 1:1 USA/Japanese fight, a 1:1 British Empire/Japanese fight, a major continental campaign, mostly historical with greater aviation development, etc.? Then you can stitch together the plot line by pulling the appropriate alternate history levers. Western Citadel falls into the general heading of an unrestrained arms race. The alternate history levers then give you the opportunity for some "color"