I wouldn’t recommend trying to stack either Korea or Manchuria. It seems to me that if Japan is investing alot of resources defending Manchuria that could be going towards China or India, then the Russian strategy has already worked and Japan is in bad shape.
Note that the Russian player pays a price when they violate the Mongolia pact.
It’s a good idea to try to position your fighters after J2 so that as many as possible can reach Manchuria/Korea on J3 (your goal is dissuade Russia from stacking Manchuria and have a few aircraft that can reach Korea too).
That said I wouldn’t be too worried even if Russia stacks Manchuria. Your focus on J2 and J3 should be rapid expansion and NOs, not Russia/Korea. But if Russia continues south to Jehoel on R3 you might have to attack J4…but even if you don’t (hopefully because there is a better attack to make) it’s not the end of the world. If Russia stays in Manchuria you’ll need to destroy that army eventually, but it’s easy to concentrate alot of aircraft and transports on Manchuria once you’ve already achieved your other objectives.
The danger that you need to block at this stage is a USA landing on Korea reinforced by the Russians.