That is indeed a tough one to wrap around.
I think part of the problem is that on the Pacific side of the map, Japan has no pro-axis neutrals, or “activation” rules for units in the territories of Manchuria and Korea. These territories and defenses must be essentially built up and maintained, just like they were in the actual conflict(s). In short, the current set up prevents Japan from using all her forces, without the consideration that she would leave herself open to attack.
You could do two things: Hope for the best from the Bears in North, and if truly necessary, counter and attack when the time comes.
Or, perhaps house rule Japanese puppet states like Manchukuo aiding in the territories defense.
I personally don’t agree with the latter, but it is just a thought.