I think that the problem I have with your thoughtful analysis is that the UK at this point had basically conceded that it was incapable of defending Singapore and Malaysia in the event of an European war. And in 1939, the UK's primary concern was with Europe--while at the same time the UK felt that
The UK believed that its colonies in the Fear East were vital to its economic welfare. (Modern economic scholarship sometimes argues otherwise, but that's another issue.) Certainly, the Far Eastern colonies were important to UK economically to the UK than the Philippines were to the US. Loss of the Philippines would have no real economic effect on the US, per se. (Though a situation where the Philippines were taken would.) The US strategy--not unreasonably, given this--was to retake the Philippines when strategic superiority rather than to race into the Philippines. Even without a war Europe, the UK had considerable doubt about being able to defend Singapore and Malaysia--and for good reasons.
Fair comments, and the relative importance of the locations does shed new light on "best".
I'm uncertain that Malaya is definitely indefensible in the case of a European war - I think it depends a lot on the nature of the war and the timing of the Japanese attack. While you'd expect Japan to attack at the most advantageous time, faced with the choice of a suboptimal opportunity or no opportunity at all, they may still go south. Certainly there are cases of where the troops and aircraft cannot be spared from the European front to be sent to Indochina and Malaya - if we imagine Germany crushing part of the BEF or French Army, resulting in a manpower crisis, for example, or simply heavy fighting that sucks in too many reserves. Alternatively, if a German attack fails with heavy losses - such as Blunted Sickle - then there may be the forces available to be sent east, at the cost of weakening the offensive against Germany, which may be deemed acceptable.
We know historically that trying to hold Malaya without Indochina and while fighting in Africa and supporting the USSR was impossible. But how many of these three factors need to be negated to give Malaya a realistic chance? With the historical forces available, I'd say that Free French Indochina is insufficient - Japan will just roll over it and Malaya more slowly, because of air superiority and the ability to outflank by sea. But adding in the North African forces in the case of a neutral Italy may be different. The Japanese forces that took Malaya were not particularly strong, but I have little idea of how quickly Japan could build up strength in northern Malaya to resume the offensive if the original campaign stalls halfway down Malaya.