First of all, I want to say that USA committing any purchases to the pacific is a mistake. For the same amount of resources, the USA can always pressure Germany more than Japan. This is from my experience playing hundreds of revised games, many at a high level.
That said, assuming the US commits to the Pacific…
1. Japan should always sink the US fleet at pearl harbor. If this is done, Japan generally starts with a 1 battleship, 2 carrier, 3 fighter advantage compared to the USA that will take 5+ turns of buildup for the US to overcome.
2. The US can’t make significant advances unless the Japan fleet is destroyed. The best way to stall the USA is to place your fleet in the sea zone off the phillipines or china and deadzone all sea zone within 2 spaces of Japan’s fleet. Have the carriers filled with fighters AND try to have an equal number of fighters in a land territory that can reach all the sz within 2 spaces of Japan’s fleet. For example, 2 carriers can support 8 fighters attacking a sea zone.
3. Japan can continue pushing land units into asia, even with all US purchases going to the pacific. My typical purchases are like 6 inf and 2 fighters. With the Japan fleet off of China
From what I have seen when the allies does a KJF is that, PH really isn’t that easy. If UK commits and is lucky, PH can be very difficult. Complete UK commitment would be UK india ftr attacking the J sub in 45 with the australia sub, and then landing on PH. The Australia TT taking 2 infs to papua, the indian TT taking 2 infs to borneo, The UK CR attacking the japanese TT in 59 and the India CW either standing in 36 or 49. At this point, taking PH might be difficult for japan. If they do it with air only (5 ftrs + bomber) they will lose about 3 ftrs for 2 ftrs, 1 cw, and 1 sub. The good news is that they will not lose boats to a counterattack, and can use their boats to deal with the british fleet, the bad news is that they are spread out, will be in trouble on land and will take 3 ftrs in losses for only 36 US ipcs (+ 10 UK). If they try to do a heavy PH, alot of the uk fleet survives and UK will keep borneo until J3.
I still feel it is a bad move, since you are handing africa to the axis, but japan will be in trouble.