That question kinda hurt my head to read.
You have a head?
The rulebook doesn’t say to resolve all air combats first. The section you are referencing is about air units flying over territories with antiaircraft guns.
Simply, you do NOT use a Jap fighter and infantry against a Russian territory as two separate attacks in the same turn. They are considered ONE attack, and the attack triggers Nonaggression treaty. If you attack with just one fighter, that triggers it. If you attack with just one infantry, that triggers it.
The only real ways I can think of to counter Nonaggression Treaty are to attack in overwhelming force, to attack with something expendable (like one infantry), or just ignore Russian territories in Asia and press on India, China, and Ssinkiang (when Russia walks in through the open door, you counterattack).