Well, the destruction of so much of the US fleet in the sabotage allowed them to make their surrender (which was going to happen anyway) a bit more conditional. The atomic bombing still changed their public consciousness and culture, but they managed to retain their military might and avoid US occupation. Between these two things, they managed to rebuild their economy much faster and restore their naval power. Add on the massive shock of 160,000 lives taken in one day, and Japan was now morally conscious with a good reason to be angry. So after give decades of cooling off what we have is a morally conscious naval power that has been nuked in the past and hates the bomb with a flaming passion. Not much of a stretch that they'd take action when the US subjects Europe to nuclear bombardment and tried to leave without fixing any of it.
I think it's kind of a stretch to be morally conscious and a military power descended directly from Imperial Japan, I think you need a coup or some other change to get rid of the pseudo-bushido mindset. That Japan is a bigger power and the US maintains its anti-Japanese sentiment makes more sense. In your initial writeup it wasn't very clear how wartime and post-war Japan's trajectory was different other than strategic bombing from Tinian stopped (because it exploded). With that in place, it makes more sense.
It's a public cultural shift, not a military cultural shift. (At least, not at first.) The old, stubborn jackasses lose their influence over the younger generation and eventually step down or die. Now you have people who are younger, less stubborn, and impacted by a nuclear bombing who fancy themselves as holding the US's leash. The military is much as it was, but politically the nation is more progressive and is making a large number of "moral" changes and socially it's completely different.
I think for your characters to really be powerhouses, you need to elaborate a little more on how they got so powerful, since in real life owning just one aircraft company or arms factory, or even oil company, wasn't that big of deal. Hughes was competing with Lockheed, North American, Grumman, Douglas, Northrop, etc in the aircraft market, for example. I think some mention of their secret takeovers of their competition or something like that would be good.
None of them really were. They just were large enough and smart enough that in tandem they managed to pull a fast one on the US while it was distracted by the USSR. It didn't require as much in this setting, just a bit of ingenuity and being the first ones to act on something others hadn't noticed yet.
I'm also writing setting info as we speak. Might take a couple hours.