The kind of scenario that Johnrankins is describing sounds like something that USSR would try to do, and even they, a totalitarian country, couldn't pull it off--very few Russians might not know "the truth," but they know darn well that their government habitually lies to them. Conspiracy theories run rampant. Governments have trouble building consensus. Not exactly something to build a well-functioning government that commands trust of the public and makes politics safe for demagogue wannabe-strongmen--who can only pretend to lead, because very few people would actually risk their necks following them. Not exactly a setting where Truman, Marshall, and Eisenhower could lay down the foundation for postwar international order, it seems. If USG has to resort to this sort of business as consequence of Japan not engaging in direct aggression against US, that seems to be the departure from OTL dwarfing anything that ever happened during WW2 OTL.
I am not suggesting any such thing. I am suggesting there would little or no evidence to prove otherwise. The large majority of the Japanese involved would have been killed during the war. Hell , most of them would be killed in that battle. Few, if any, documents would have survived. The handful of documents that might survive would simply disappear.
The immediate post-war Japanese government (which was terrified by the Russians) might well be the ones that disappeared them and "admit" that they were trying to invade the PI. So you would have very few witnesses, many who might have been unsure where they were going anyways (I am talking about the low level troops here. Information was pretty restricted for the average Japanese grunt IIRC. ) as they probably not be told much until just before the invasion over the fear of spies. On top of that both governments would likely be saying the PI were the target.