If the overall strategy is to fall back to Singakirk/Dunkapore the additional training and leadership will not change the eventual outcome. The men know that. A winning strategy is not there yet.
The British see Malaya as a land campaign. So do the Japanese, at least initially. It becomes a littoral campaign when the flanking amphib-lite maneuvers occur in the South China Sea or Strait of Malacca succeed. That's how I see it unfolding.
Do the ABDA meetings pre-war address this issue, particular to Sumatra?
You are mispresenting the strategy even of OTL. It was, if Perceval had not dithered, to be in three phases. First launch Operation Matador into Thailand. This would have slowed down the Japanese if successful, the next step was a bit like the Soviets wanted to do (and the Russians had done in 1812), bleed the invaders whilst trading ground to keep forces as intact as possible. All done in the expectation of reinforcements arriving as well as the Monsoon. Last phase would have been to counterattack once sufficient force had arrived, after the Monsoon, driving the invaders back. In no case was it supposed to be, fall back to Singapore and fort up.
Now OTL the Japanese shortcut this by a combination of being able to advance at high-speed, disregarding logistics and a lot of luck in capturing supplies/boats/key locations quickly. Troops that are better trained, know jungle is not impenetrable and have tactics to counter infiltration, will slow the Japanese up. This makes the wheels fall off, if the Japanese don't get to Singapore at OTL speed (as they were all but out of supply by then), they have no option but to pull back to a position they can get decent supply to and pause. This would almost certainly mean the Monsoon stops play and the momentum shifts. (It also means the Burma Campaign almost certainly does not happen as its supplies will have been used to try and force the way to Singapore. So, no Bengal famine and the Japanese having to guard against an attack from the West)
As for the amphibious landings, these need two things the Japanese are likely to be short of, boats (most OTL were captured ones rather than part of the Japanese force so slowly advance means less are likely to be found) and a panicking, lethargic unprepared opponent (a competent commander could easily block the assaults as they would not have much supply or heavy equipment, let alone use proper landing craft). Remember also the longer Singapore holds, the more submarines are going to cripple the Japanese logistics which are marginal to start.
The Japanese cannot just attack Sumatra, they lack everything. OTL the attack was launched after Singapore fell, mainly using forces that had taken Singapore. This because as long as Singapore holds, its near impossible to attack from anywhere but Java. However, the only way to take Java whilst Singapore holds, is by marching across it from the East. Which then brings up the big flaw in the Japanese concentric attacks plan. If they are still fighting heavily in Malaya and so using up all the supplies they can get hold of, they will have very little supply left over for the Central attack which is needed to reach a position to even attack Java from the East.
As you can see, once the Japanese start getting behind schedule or take heavier than expected losses, it quickly becomes a total train wreck. Their plan only works if everything goes right, which it did in OTL. Once things go wrong, it collapses as forces from earlier operations are supposed to be used for later ones. This also is the case for the logistics which assume the fighting has stopped in the earlier operation and so most of its supply operations can be transferred.