The are two plans at work here.
The strict neutral strategy should not work against a good player because of all the extra infantry (18+) that the Allies get. But those infantry take a while to get into battle. The troops is South America take at least a few turns to get into the fight if there are no transports handy in the Atlantic. It may even be a turn or two before all the extra income comes on line for the Allies as well. And the Axis get to decide when to attack the neutrals, so they have initiative. I have never seen the Allies attack strict neutrals.
As far a turn 1 attack by Japan on the U.S. - Japan can defend Hawaii and the DEI for a least a few turns from the Western Allies and the Americans. Japan can build a Aircraft Carrier turn 1 with a Transport. Attack Midway turn 1 with Infantry and Artillery. Attack American Fleet around Hawaii with 3 Fighters, 3 Tactical Bombers, 2 Bombers, and 1 Destroyer and 1 Submarine on J1. Turn 2 attack Hawaii with 2 Infantry, 2 Artillery, 3 Fighters and 3 Tactical Bombers. If any Destroyer or Submarine survived the turn 1 Assault on the sea zone around Hawaii they can help defend the fleet from the scrambling aircraft at Hawaii, or defend any American Assault on U.S. 2. There should be 4 Aircraft Carriers with planes and 0-3 support craft (Destroyers and Submarines). This should be more than enough to hold off American attack for a few turns. If there are any losses on the aircraft on any of the battles fly new planes to land on the Aircraft Carriers in the non-combat movement phase. The rest of the Japanese fleet, 2 transports, 2 Battleships, 2 Cruisers, 1 Destroyer and 1 Submarine can take Philippines turn 1 and can take Malaya turn 2. It can move on to capture the islands on turn 3 and later. Once the Philippines are taken, planes can be flown from China to cover the fleet at an airbase. The Japanese have enough fleet to match the British, Anzac and the Americans for at least a few turns. Japan does not have enough resources to fight China too while this is happening. So the Chinese will start producing 18 + income a turn and dominate the mainland. But this is the strategic problem Japan has always had. It has the initiative and not enough resources. IF you have to choose which Ally gets more money, choose the Chinese, as they cant leave China (except for Hong Kong and Burma).
All I ever see the Japanese do is build for a few turn or two, attack China turn 1 and 2, and then fight a traditional war against the DEI and try to hold on…