I've been playing well into 1941 as the United States. I must say I really love how you've expanded the GEM to include scenarios, and especially more modern ones.
The large number of nations is a bit of an issue, since I usually play in windowed mode, but I had to convert to full screen to be able to see everyone's scores. Still, that's not a big problem.
The United States was incredibly powerful, probably as you intended. I haven't declared war on anyone yet, but I did take over the minor nation civs in the Americas. On the first turn, all of the Latin American civilizations were willing to vassalize to me, which I think is a good thing. Brazil (and I believe other nations) joined the Allies late in the war anyway, and US economic imperialism in latin america can hardly be overstated.
Overall, as I said, an amazing scenario, and there's very little to change. I especially liked the use of super-promoted units in key areas, and the change to merchant specialists that allows cities that are realistically huge but in civ4 are usually constrained (like New York) to still grow very large. The extra hammers on sea tiles deserves a shout-out, as well.
However, there were (as always it seems with civ4) some issues with the gameplay of the AI. Germany never took over a single city in France (only the Netherlands), and to my knowledge Italy never took a single city either; similarly, there's been no activity in the Winter War, so most city conquering has been in China. By the gain of great generals on both sides in Europe, there's clearly fighting going on, but a more historical outcome/more action would be great.
I like how there are huge diplo bonuses and penalties, to make it impossible for nations like the US and Britain to go to war; but even with this, surprisingly few wars broke out after the ones preordained at the start. A bunch of nations declared war on poor Norway, but other than that ridiculous phony-war dogpiling, there hasn't been any Soviet-German War, or Japanese invasion of Indonesia or the Philippines, or even minor conflicts. Japan didn't even take any cities in Indochina, whereas amphibious invasions are supposed to be fixed in BtS.
Still, my guess is that the AI stuff is simply due to the game engine, so by all accounts you've made a truly excellent scenario. Keep it up!
Two last comments- I initially thought it was annoying that India was a vassal and not fully British, but realized it makes sense in a scenario where India becomes independent only 7 years after the start. Still, in my game India broke away in early 1941... maybe make them a bit weaker and more compliant? They were very very high in score, around 4th I believe. Reducing India's wealth/population would probably be bad, since it WAS the jewel in the crown, but perhaps give some of the coastal cities to the British?
Also, maybe the minor civs should start at peace, if possible, like independent cities in Rhye's mod? It was a bit annoying that Italy directed their attention to conquering the minors instead of invading Egypt.