What about the affect on the campaign?
I see two additional problems with this scenario. First, based on what old and fallible memory, historically the Japanese did kind of a pincer movement on the Dutch East Indies. One of the two pincers went through the southern Philippines once US forces were pinned up in the Bataan peninsula. The other pincer went through Malaysia and Singapore. I suppose the Japanese could have gone directly from Indochina into Borneo, but that leaves Japanese forces in Taiwan out of the equation. Also, an invasion force going directly from Indochina to Borneo would be covering a much longer distance--probably a good 600 miles, which would make land-based air cover problematic.
Second, the US could do a lot of things short of war to make life difficult for the Japanese. If the US decided to be 'neutral' in the same way they were in the North Atlantic, they could track Japanese ships and report the locations to the British. They could send large amounts of Lend Lease directly where the British needed it, and if the Japanese tried to stop the shipments then that's an act of war. The US could build up Wake, and then begin a buildup in Guam. The Pacific fleet could maneuver aggressively in the Central Pacific, tying up Japanese assets to keep an eye on them. The US could send more pilots and planes, including bombers, to the Flying Tigers, who were constituted before the US entered the war officially. They could, and would build up aggressively in the Philippines, as several people have pointed out.
In terms of Lend Lease, historically the US apparently suspended Lend Lease for about a month after Pearl Harbor to rebuild defenses in Hawaii--seizing planes that would have been bound to the British, and replacing them with later production. Without Pearl Harbor, presumably those planes would have gone to Britain, and probably to the Far East. I doubt if they would be enough to make much difference, but the Brits would have had at least some additional equipment minus Pearl Harbor.
Bottom line: In addition to the problems already mentioned, the Japanese would have had more difficulty creating a second pincer to attack the Dutch East Indies, and the US would have had a host of options short of war to make the Japanese miserable and provoke the Japanese into an attack at a time of the US's choosing. That doesn't necessarily mean that leaving the US alone was a bad idea. It just means that it had some additional downsides.