Before i give you my opinion on this, here is the Alpha 3 rule regarding this issue.
++++Amur – Manchurian/Korean Front:
With the recent unsuccessful Japanese invasion of Eastern Mongolia (1939), and the fragile treaty that followed, Mongolia feels obligated to consider any Japanese military movement into Amur as a flanking threat to its national sovereignty and will immediately renounce its neutrality and join the Allies. If the Japanese attack Amur from Manchuria and/or Korea, all Mongolian territories (Olgiy, Dzavhan, Tsagaan-Olom, Central Mongolia, Ulaanbaatar, and Buyant-Uhaa) are immediately placed under control of the Soviet Union in the same manner as though the Soviet Union had moved land units into a friendly neutral territory. These territories have Soviet control markers placed on them, and their standing army units are placed on the board and are controlled by the Soviet Union player from then on. This occurs regardless of the state of relations between the Soviet Union and Japan at the time of the attack. It should be noted that no penalty or bonus is awarded to either side if the Soviet Union attacks Manchuria or Korea.++++
Now, i admit i didn’t think about all your points for hours, but after a brief examination, here are my thoughts:
Your 1st proposition seems to violate the spirit of the Alpha 3 rule in that regard : Japan can only take 3 territories (by sea) if they don’t want to trigger Mongolia. So in effect, all easterns russian territory get to " defend themselves without troops"…
Your 2nd proposition come with so restrictive conditions, that most of the games will never met any of them, leaving the game with the complete impossibility for any figth for the eastern russian territories.
If i had to chose between your 2 proposition, i would choose the 1st one for sure.
Now, here is another proposition of mine. It’s not flashy, but i think it preserves the game balance.
If Japan attacks any russian territory, Russia receive immeditately 12 IPC, as in the Alpha 2 rules.
IN ADDITION of the Alpha 3 rule.
So, if Japan never attacks russia from Man or Kor, the Mongolian territories never get triggered.
If Russia attacks Japan, Japan receives immediately 12 IPC as in Alpha 2.
I, of course, might be totally wrong on this, but i see the Alpha 3 New rule about the Kor-Man / Amu front as a reaction to the Japanese turn 1 attack with all units from Man and Kor to Amu, and Sib with 1inf, 1tnk. By turn 2, Japan has secured a solid 5 IPC in territories that they might hold during all the game (in fact, this 5IPC most often becomes a 7 IPC/round).
This pattern was becoming the standard Japanese opening and was very strong.
Now, with Alpha 3 rule, it’s more complicated to do.
What do you think ? Feel free to propose anything else if you wish or to modify your propositions as you like.