Using starting inf in Manchuria and Korea to hammer China early

Just an observation: whether they are in Buryatia, Yakut, Yenisey, Timguska, or Novosibersk, the Russian far east units can always march to liberate Kiangsu in 5 moves without ever being on the coast, should Japan break the Mongolia treaty.

Edit: so anytime Japan breaks the treaty within the first 5 rounds (when the Russians are probably in those territories), they then have exactly 4 turns to achieve a Pacific VC win, or it may be denied by the Russians liberating Kiangsu within 1 round.  Once the Russians and Chinese are in Anhwe, Japan will have a very hard time getting the VC win and killing off those communist forces.

I actually like breaking the mongolia treaty as japan… then my units can move freely between the spots instead of being divided on the other half.

As Japan you cannot let RUssia walk into Korea or Manchuria, which would happen if you send all your troops deep into China. I agree sitting there waiting to be attacked is equally silly.
What is the solution? Attacking is depressing, because you will have enough enemies in 2 or 3 turns and killing 6 Inf(if they stay) gets you nowhere because by attacking Russia you add 6 more to the equation! Attacking only works if the Russian player moves all his Inf into the same territory and you have the air  and ground units to wipe them all out. You would be lucky if he gifted you that!
I find a horrible and Japan killing stalemate ensues. As Japan this is unacceptable.
As Cow has said, I would  attack the Russians. Unlike him, i would not DOW on the Allies. This means you are hoping the European Axis will win, as Japan will never get rich on Chinese and Russian pickings, but if you do not attack you spend the whole game looking over your shoulder expecting a Russian attack in your weakened rear!

I am very different, I try to pursue a pacific win in all my games. It rarely happens though. If it does work it, it usually is in low luck games sadly. I do ignore russia for some time as japan typically… after all the plan for germany usually is to take it down sooner than later.

What Japan does in the north really depends on what Russia does.
If they pile up all 18 infantry on Amur, then do like Wittmann said, pound them good and quick. Except for the 6 guys in Mongolia, that basically opens up all the Russian far east territories for Japan to walk into over the next few rounds. Almost like free IPCs for Japan.
If Russia withdraws all troops to head for Moscow, which I have seen happen, that leaves you free to use most of those troops in Manchuria and Korea in China or other places you might need extra troops. Just leave a few there on the border. Then after 3 rounds or so, when the Russians are too far west to hinder you, walk some troops up and start taking Soviet territories. Of course, you will have to deal with the 6 Mongolian/Russian troops but that shouldn’t be too difficult.
If Russia only withdraws some of the far eastern troops, like say he leaves 6-9 infantry in Amur and heads the rest toward Moscow, that usually means he wants to guard his eastern territories but isn’t looking to be aggressive toward Japan unless a blatant opportunity comes up (like Japan withdrawing all troops from Korea or Manchuria and leaving them totally open). In that case, as Japan, I would simply leave the troops there on Manchuria and Korea and consider that area a “non-combat zone”, meaning there is simply no fighting going on there. You could possibly take a couple of the troops to use elsewhere, but you don’t want to weaken youself too much or Russia might take advantage. Yeah, it sucks leaving so many guys just sitting there, but you have to keep Russia honest. Plus, at least you start with those guys and any new stuff can go to your campaigns elsewhere.