<snip>However, if we end up forcing USA to split their involvement over both theaters, which seems to me to be a losing proposition nearly every time, aren’t you then imbalancing the game toward the Axis?</snip>
I don’t think it is a loosing proposition nearly every time, so I disagree with your premise.
It’s a game opener more than anything else. Remember originally - the stand alone games were balanced with a half USA in each. If you then combine the boards and throw the USA into one side of the conflict, you’re effectively putting two countries into one side. And the other allies basically just need to avoid loosing, and the game is won. India doesn’t need to win the game, they just need to not loose. Russia doesn’t need to win, they just need to slow down the axis enough. And so on.
I don’t see many other viable choices to a balancing act upon USA to be frank.
By buffing/nerfing the other axis/allies you’re effectively removing strategy and closing options by making alternatives too easy. If it becomes easier to take Russia by Germany/Italy for example - nobody will go Sealion. Strategy removed and game made more narrow.
By giving USA a clear incentive to split attention somewhat (I’m not saying 50/50 product limit or something) you open up the game and provide the chance for more strategies. If the USA can’t dump 2 battleships, 1 crusier and plans into the Pacific each round at war without loosing something on the Atlantic side - it will provide more possible strategies for the game overall.
A clear benefit as I see it.
Tweaking the USA is a win/win in my opinion. It makes the game more balanced and it will provide more choice for how to play. All other options as I see it might balance the game, but will remove choices.