Thanks for the comprehensive reply; I excerpted only the sections I had follow-up questions:
On soft build limits: One of my main concerns in the later - or even middle turns - is that the AI unit spam would get out of hand. I have seen games on Youtube where the AI replaces units faster than the human player can destroy, and it results in the AI having excess of 10k war chest in the end. Wouldn't soft build limits make the AI spam that much worse?
I haven't had that problem. If anything, the AI seems less effective when building new units with soft build enabled. The Land posted (on this thread)...The AI doesn't really understand soft build limits - it appears to make poor choices with them - building very expensive units above the limits when similar units are available within the limits. So this option hampers its efficiency somewhat. So that works to my advantage as well!
On the Chinese terrain: For this very reason, I am thinking researching Armored Warfare quickly and plopping down tanks in China. They have more movement than infantry and can attack multiple times - preventing wounded Chinese units from retreating and replenishing. Would this work?
No. Tanks need roads/open terrain to move about. They can't go up mountains. I did use tanks in China, but mostly in the north where there is some suitable terrain. Even then it was heavy going because of stretched supply lines etc.
On playing only one nation: I asked the same question in this thread to someone who recommended me to do the same: Wouldn't this disadvantage you more - especially as Axis? It seems like there needs to be some coordination for the Axis to win, and I am afraid that the other Axis powers will go to hell if I just control nly one - even if I am a newb. How did you find that the AI performs as "partners" with you?
Yes, controlling all of the Axis powers should give you a big advantage (IMO). Having said that.....
It will extend the game turns and you will be spending a LOT more time planning/moving etc. Keeping track of where everything is becomes difficult. Much easier to control 1 nation for a new player. It's the best way to learn the game mechanics without getting bogged down in detail.
Also, although I like what-if scenarios heaps, historically this just didn't happen. Italy tried to wage a separate war in the Mediterranean (until they lost all credibility) and Japan didn't help with Barbarossa because they were obsessed with the Pacific.
I intend to try this what-if myself at some stage, but it's too big/unwieldy. Will continue playing 1 power for now.