Thinking intuitively, I don't think Hitler nor his generals would've put much hope into Japan to actually do much against the USSR. Japan was a sea-based power, and did not have the developed mechanized force needed to effectively fight Russia in the expanse of the far east. The SSJW for instance was almost entirely fought by infantry.
In regards to the OP, I'd say that a stronger Manchuria defense could have many outcomes. It could lead to earlier Chinese unification since the KMT successfully driving back the Japanese would showcase them as heroes. OTOH, Jiang Jieshi could get lazy and make more mistakes, leading to a prolonged warlord/anti-Communist struggle, and we could see Zhang Xueliang stay in Manchuria and form an independent nation there with all his resources. I've always been interested in the idea of an independent Manchuria. I think that in the modern era it could be a very successful nation.