I stopped just after Barbarossa, and I missed some of the early stuff you might be looking at us checking, but here is what I saw over that time:
I didn't play with the intervention slider, I tend to just leave sliders alone when I play the USA. As for the manpower, I suppose you're probably interested in post-war manpower, so I can't comment on that.
Republican Spain won the Civil War, no crash or problems associated with it (as far as I could tell).
AI didn't seem to trip over the new provinces (at least those in France), and acted more or less as expected.
The most significant problem, which caused me to end up stopping early, is that the AI UK was unable to keep up with the Lend Lease events. Early 1941 Germany put a small force on the German isles (just south of Edinbourgh), some 6-9 divisions although they expanded that landing over time, and took over the Isles without any problem.