Along with the optional rules for USSR and US entry, perhaps the victory point targets should also be flexible. I suggest having the targets start the turn after both the USSR and US are in the war. That way Japan is not forced to attack the US in spring of 42 in order to include Japanese victory points into the Axis total to prevent a defeat. (An item that Yoper brought to light in our Big Game in April)
The other effect is that if the USSR and US enter early, the victory point "clock" if you will, starts early. For example, if the USSR and US both enter the war in autumn 41, then the victory point targets would begin in the winter 41/42. They would then count down at the same pace ending in turn 24 rather than turn 25. The opposite would then be true if the USSR and US enter the war later than they did historically.
The USSR seems to have the most of the control over when they are ready to go to war, while the US seems to be very dependent on Axis actions, especially Japan's. Just an observation.