I wonder what the four old fourstackers could add to FLAK defense, with their non existant AA outfit. (Just 2 mg's at best each ship.) Better have these operate independently as a light hit and run group, or delete them entirely. Also, NO SPITFIRES in 1941 outside the UK and Malta, as the Spitfire was still urgently needed in the UK and not being send abroad. Simmilarly, the IJN had no carriers likely here, as there was enough landbased airpower nearby, besides the carriers being needed for other tasks, such as buidling up airgroups and support over the Philippines.
Likely, the building up of British forces would force the IJN to allocate more strikeforces in the Saigon erea, with a second airgroup, which in the OTL was stationed in Formosa. These would be the main weapons, besides the likely increase in numbers of submarines in the Malayan region. Kondo would then likely have had fewer ships in the consequense, as the IJN was not so stupid to waist his forces against seemingly superior numbers of the opposition. More likely he would remain in the back, for a possible show in the aftermatch, as is trained for by the IJN in the years prior to war, but mainly against the USN. Also the sailing of the convoy is postponed, untill the sting was out of the Allied attackgroups.
What is likely a scenario?
Submarines strike with more force on the Allied ships, possibly killing both the large carriers in these confined waters, where they had nothing to do. Also, the order would have been to act more agressive in line with the IJN Submarinedocytrine against large surfaceships, so it is likely one would have attacked the capital ship force as well. With no aircover again, the result would be simmilar, or even worse, with all larger Alied ships sunk, or damaged and no losses to the IJN ships, besides some possible submarines and a small number of aircraft.
Sir in real timeline DesDiv 57 which consisted of old destroyers USS Alden (DD-211), USS John D. Edwards (DD-216), USS Whipple (DD-217), and USS Edsell (DD-219) were order to Singapore to give AWS (Anti-warfare submarine) because they had good sonar. You are right about anti-aircraft guns they were obsolete but against submarines they were really trained for the job. About the spitfire, well it is true what you say but hey its a alternate timeline and basically this scenerio would have Singapore and Malaya really arm to the teeth.
About the carriers, I added them because I figure that the Japanese find out by spys that two carriers were heading for Singapore by 28 of November and so they send this carriers. Remember the Japanese wants war and they will do anything to conquer Malaya and Dutch East Indies oil at all cost. Seening that this force is forming maybe they remove ships from the Philippines theater and send them to Malayasian Theater.
About the submarines well thats where DesDiv 57 will come in and try to attack them. That was my thought for this. But still How would a refit HMS Hood come out of this?