2) Where are they going to invade from? OTL they came from Siam and Indo China. Siam only happens because the French are forced to give up Indo China. So the Japanese are coming from Formosa, with no land route to Malaya. And they have to take Indo China from southern China. That is a little more challenging.
Now you say "Assuming that the Japanese aren’t heavily involved in occupying China". Why the heck are they invading the European colonies then? No one is going to be worried about Japan and Japan will be making nice civilian money hand over fist. No China no problems.
3) About the only experienced units were the Navy ones that came with PoW. I would have to check for the land troops for who is who.
From Wiki
"Indian III Corps was formed in mid-1941 as part of the
Malaya Command when the increase in tension in the
Far East necessitated the dispatch of large reinforcements to the area to deter
Japan. On 7 December 1941 the Corps consisted of the
Indian 9th Infantry Division, commanded by Major-General
Arthur Edward Barstow, the
Indian 11th Infantry Division, commanded by Major-General
David Murray-Lyon, a
lines of communication Area, and the Penang Fortress.
[4] Due to the rapid expansion of the
British Indian Army, many of the formations in the Indian divisions were ill-trained and lacked large enough
cadres of experienced troops."
"The
8th Division was an
infantry division of the
Australian Army, formed during
World War II as part of the all-volunteer
Second Australian Imperial Force. The 8th Division was raised from volunteers for overseas service from July 1940 onwards. Consisting of three infantry
brigades, the intention had been to deploy the division to the Middle East to join the other Australian divisions, but as
war with Japan loomed in 1941, the division was divided into four separate forces, which were deployed in different parts of the
Asia-Pacific region. All of these formations were destroyed as fighting forces by the end of February 1942 during the
fighting for Singapore, and in
Rabaul,
Ambon, and
Timor. Most members of the division became
prisoners of war, waiting until the war ended in late 1945 to be liberated. One in three died in captivity. "
So yeah. Newly raised troops that never made it out of Asia, vs China veterans. Not exactly "originally taken part in the war against Germany and Italy." Which is completely logical. The War was being won or lost in Europe and Norther Africa. Why would the good troops be in Asia?