I don't think anything could have deterred them. The southern offensive was "do or die" for the fascist leadership and they were determined to go all-out. The decided on total war against the United States, after all.
For anybody proposing to deploy additional troops in Malaysia, do not forget WWII supply requirements (per division):
1) Survival - 10 kton/year
2) Training/re-organization - 25 kton/year
3) Medium-intensity combat - 100 kton/year
4) Heavy combat - up to 220 kton/year
Moving division itself is actually the smallest part of the logistical problem.
What this requires is the world's largest Merchant fleet
Oh look found it
This works for the Japanese as well - increased Sub activity in the South China sea + Gulf of Thailand and perhaps Adm Hart actually using his Subs offensively around the Phillipines would soon see serious difficulties for the Japanese Merchant fleets ability to supply the IJA 25th Army.
Its fighting a defensive battle on a battlefield it has controlled for years so as long as the supplies are generally available to GB then it shouldn't be a problem ?