Back to the original question, how best to attack the US from Japan? Well I would not try to attack the US with Japan initially as it is very unlikely to work. However, IF I were to do so, I would do the following:
J1) Take out the US Fleet at Hawaii, buy inf and transports.
J2) Go after the remaining US fleet with BBs and subs remaining after Pearl and take Alaska with transports and AC. Buy inf and transports.
J3) Take Hawaii with AC, one transport from Japan, and remainder of Japanese fleet (destroying any builds by US on West Coast with aircraft from AC. Start a “shuck-shuck” to Alaska with the transports.
Basically, I would start moving troops enmass to Alaska every turn. The reason to take Hawaii is 1) to provide a base for bombers from Japan. They can assist when you are ready to attack W. Canada, W. USA, strategic bombing runs on the W. US coast, or if the US builds a fleet off the W. US. 2) To move later to take Panama, or Mexico, (and how about Brazil?) to harass the US. and 3) To increase your ipcs while reducing the US ipcs. Also later turns can be used for side trips to Australia and N.Z. to increase the Japanese ipcs.
But my main focus of the attack would be through Alaska. I would hope the US would not be able to take back Alaska (perhaps they wouldn’t have the forces within reach so soon) and that Japan could reinforce Alaska faster than the US could put forces in W. Canada (unlikely). I would perform strafing runs from Alaska to W. Canada using bombers from Hawaii and the fighters from the AC (and perhaps from Alaska) to assist, until I reached the point where I could take and hold W. Canada. Then do the same to W. USA (watch out for AA) and/or E. Canada as the situation permits.
It might be possible to have some sucess in the early game here, but I think its best to attack the US with Japan once/if Russia falls because at that point Japan probably has a larger economy that the US and doesn’t need to defend its Russian front.