Yes, a campaign of delaying and attrition is all that is needed to really throw a spanner in the Japanese works. If they can't grab Singapore in one swoop like OTL then they have to pause for logistical reasons, and that opens up a can of worms. It's for that reason alone that I think a real go-getter of a CG from April '41 would be enough to tip the balance. However I think April may be too early for the ideal CG, because I think such a man would be best schooled in Norway, Ethiopia, Greece and/or other shoestring campaigns, but these are too late for convenience.
My vague concept is for our hypothetical general to spend 6 months whipping his forces into shape, and modifying his plans based on what he has. So Op Matador could be launched in brigade strength on time, unlike the delay and change from division to battalion strength made at the last minute IOTL. And he doesn't ask for Force Z to put it head into the lions mouth on the first day of the war.
The next event in my vague concept is in mid Feb. The intial Japanese campaign stalls a considerable distance from Singapore for logistical reasons. Force Z/ABDA naval forces intercept and destroy/scatter the Japanese invasion convoy headed for Sumatra. So in mid Feb southern Malaya and Sumatra are in Allied hands and will remain so for some weeks. I think this is the key to my concept.
The third event in my vague concept is the arrival of Sommerville in early April. I've yet to work out what's possible for Sommerville. However I'm thinking he won't have to shadow box with Nagumo as he did IOTL because Nagumo will be satisfied with sinking Force Z/ABDA ships that thwarted the invasion of Sumatra, and plastering the British in Malaya.
My main problem is that my TL will be dry and without detail compared to others that people put up.