@trout
Hi Trout
I think what happens to the Mongols when attacked by SSR is that they turn Pro Axis so you have to move a dude in NCM to take them over. Same as Finland and Bulgaria Iraq
Edit
So what happens at the start of Early 1941, turn 3, the Mongols are treated as a "True Neutral" which means that an attack on them would turn All True Neutrals to Pro Axis.
For this game, I would just let the Mongols and not all the other Neutrals turn Pro Axis as it's too far along to go back and redo it.
I assume Cat wouldn't have attacked them if he knew that.
So the Mongols can't move. Jpn or any Axis player, has to move in NCM to activate the TTy and any units in it to JPN control. Triplea does this correctly.
So I would move the Mongol Inf back to there starting TTys and return control to the Mongols, except for the one SSR already conquered of course 
Starting with last turn J6 JPN can then move the Inf from Chahar to Central Mongolia in NCM and it will turn Japanese 
It's important to note that triplea will allow you to move M2 units and activate more than 1 TTy. That is incorrect. Once you move into a Pro Axis or Allied TTy in NCM, you have to stop whether you have movement left or not.
AAGuns also can't take control of the TTy which triplea will allow.
Edit 2
So to sum up, OOB rules which are in affect the first 2 RDs, if JPN attacks Mongolia or any Russian TTy bordering Mongolia, the Mongols immediately turn Russian including there units.
If Russia attacks Mongolia, all Mongol TTys become Pro Axis. Same as Finland, Bulgaria etc ... triplea does both of these correctly.
In HouseRulesExpansion beginning turn 3, Mongols become a True/Strict Neutral. An attack on one Neutral turns all the other Neutrals Pro to the side that didn't attack.
Russia is only player that doesn't do that. I guess I should change it but it's a pita and it's easy enough just to edit the Neutrals to at War.
I don't necessarily like thoese rules but that's what they are 