From: St.Petersburg, Russia
So, on fleet, only a few thougts, at the moment...
Just in case: vision of carriers as the fleet's main strike force cannot arise before 1937. Even then it requires a huge leap of faith, assuming that the new generation of carrier planes existing only in specifications and on drawing boards at that moment, will be able to achieve the characteristics desired from it AND that these characteristics will overcome improvements in AA defence.
I think we need an overarching and different vision to guide changes to the fleet, and to achieve that the concept of the Decisive Battle (tm) as it existed in Japanese admirals' mind need to be exorcised somehow. I propose somewhat different evaluation of the Russo-Japanese war's lessons. The task faced by Japanese then was not dissimilar from the strategic conundrum of the Pacific War, but their post-war thinking concentrated overtly concentrated on the flashy success at Tsushima, while not paying enough attention to the fact that Tsushima was only the endspiel of the long and narowly won game, by which Japan had an advantage of such magnitude, that not only victory was almost predetermined, a tactical draw was a strategic victory for the Japanese as well, as the only remaining Russian port in the theatre had insufficient repair facilities to handle seriously damaged battleships.
So, how about postulating that after WW I and the first naval limitations, when it became obvious that any possible naval war will be a war against an opponent with both numerically superior (but not entirely concentrated in the Pacfic) fleet at the beginning AND greater ability to reinforce it, lessons of the Russo-Japanese war were reevaluated more carefully? For example (that's just an couple of ideas):
1)The war is explicitly not expected to be over after the first major defeat of the enemy fleet (while the fleet's leadership didn't honestly believed in a brief war by 1941, this idea negatively impacted Japanese operational thinking). It is assumed that the enemy will try a rematch using a combination of parts of its fleet that initially weren't assigned to the Pacific, old ships, and newly built ships, and will not seek peace until IJN's superiority is proven repeatedly. Therefore the war is expected to last no less than 1.5-2 years. This should impact approach to many areas, from sealanes protection to personnel preservation.
2)Expanding on (1), it is expected than in the opening phase of the war Japan will seize the territories that must be held or used as bargaining chips for peace, and after that an extended phase of active defense, including defense against enemy attempts to interdict Japanese communications, will follow. Attrition of the enemy fleet is expected to take place over months, not days immediately before the main engagement (so sub operations aren't tied to the operations of the surface forces as closely, for example).
3)The idea of delivering a crippling opening blow to the enemy fleet in the base is mulled over even more than it was. So when carrier aircraft, potentially capable of this, appear (see above, not before 1937), the idea of a dedicated carrier striking force is formulated faster than IRL.