Cryhavoc101
Donor
You are omitting the possible Japanese reactions. They relied heavily on local espionage (provided by locals who in majority were hostile to British cause) information, not only on documents captured from Andromedon vessel. Any buildup of British forces would be detected, taken into account and countered. Of course, Japanese did not have infinite resources, therefore some secondary operations, especially in DEI, would be delayed. But the Singapore and Malay peninsula with it, being the primary Japanese target, were pretty much destined to fail. Therefore, Percival`s defeatist stance was perfectly rational.
On his place (having all the knowledge of future), i would actually try to prepare a large-scale evacuation, may be to Java. Losses in men would be well over 50%, therefore even this move is morally questionable although makes some military sense.
I am not ommitting anything.
OTL the Japanese Logistics was sketchy and was unlikely to absorb serious shock - by the time they got to Singapore they were on the verge of eating their belts
The 3 British Divisions were understength and under equipped. Some of them were little better than Militia.
Had better units been in theatre such as those vet Australian divisions then the commonwealth forces are far more likely to attack and capture the Kra Isthumas and defeat the Kota Bharu landings (which very nearly failed OTL).
It would not have taken much to inflct far greater 'friction' to the Japanese plans to the point where the principle camapigns in Malaya and the Phillipines seriously unravel and this has a knock on effect to the subsequant campaigns in DEI, Burma and elsewhere.
An increase in Commonwealth units (in terms of units, better quality troops and equipment) would require more units and logitics on the part of the IJA and I am not convinced they could support such an increase logisitcally or without serious changes to their time table.