Actually Mongolia can go pro Axis. The only situation I can see it goes like this, some allied nation attacks a strict neutral triggering all strict neutrals to go pro-Axis. USSR breaks Mongolian Defense Pact and attacks Japanese territory. Under G40 rules, if USSR breaks the pact, Mongolia becomes a strict neutral thus in this situation, Mongolia is now Pro-Axis.
You should reread the rules, they are clear on mongolia.
Mongolia will never become pro-axis or something similar is written in the rules.