If the British & French had kept Herr Hitler under control, There would have been no need to buy from the U.S. We could have remained constrained by the depression for a few more years.
A good point. What does Japan do if a remilitarized Britain, France (if even a "contained" Hitler and Mussolini are still in power, rearmament would have still occurred) are not distracted by wars with Germany or Italy? Well before Japan's invasion of China reaches the FIC border we can expect a sizable French naval, ground and air force to sail for Saigon. Meanwhile a significant British force will ship out to Malaya and a smaller Dutch one to the DEI.
If it can be diplomatically sorted (a big if, as Italy has just invaded Ethiopia against British and French objections), I'd have Britain and France invite Italy to send a naval squadron to join the effort. Italy already has a small naval force in north-east China, see Regia Marina - Wikipedia. As Europe's Rodney Dangerfield, Mussolini is always looking for respect. If invited to join the "great" powers in this far east adventure, Italy might consider exiting any agreements with Japan. Singapore can serve to support all four naval powers.... with invoices for fuel, food and kit being dutifully issued by the Foreign Office.
Japan, Thailand (where a non-aggression pact was on the table) and various native rebel movements only became emboldened to move on FIC following the fall of France in June 1940, and the apparent weakness of the French Government. That was compounded by blockades not only imposed by Germany on military materials leaving metropolitan France but also by a British blockade. Britain sought to intercept French shipping travelling between French colonies in Africa & the Indian Ocean until the end of 1942 and Operation Torch as well as denying access via the Suez Canal & Malacca Strait..
The French already had some 50,000 troops in FIC in 1940 and a small fleet based around the cruiser Lamotte-Piquet and a couple of colonial sloops. These forces prevailed over the Thais in the Franco Thai war of 1941.
It was June 1940 when Japan issued an ultimatum to the French Governor. In Sept 1940, when Japan manufactured a casus belli and occupied Northern FIC, they didn't really have much of a land border between their Chinese conquesta and FIC itself.
If France is still independent and strong I expect the Governor of FIC will tell the Japanese to get lost. For starters, I believe the Governor of the time was a Vichy lackey, so that may not be the case. Will Britain stand by while Japan tries to roll over FIC?