To be honest, I don't know anything about the Japanese Empire or its military. But why should an army which occupies enemy territory create a new independent country and separate itself from the motherland?
It is of course possible to think of a coup or revolution in country X, after which army Z which occupies country/colony Y declares the new government illegitimate and is therefore de facto independent. Your scenario requires de jure independence.
For an independent Manchuria, I see possibilities in the Chinese Warlord Area. If as in OTL one warlord fraction controls Manchuria, we have the first step towards an really international accepted country. But we need two other great changes, that this Warlord-Manchuria develops into a real independent country.
1. We need a Japan Screw. Since my knowledge is limited, I don't know how it is possible to prevent further Japanese Influence and invasions into China. A strong Japan would either conquer or vassalize (or both) Manchuria.
2. We need to prevent the "Northern Expedition" which united most of China under the Kuomintang, since such a unification would either lead to a Chinese conquest of Manchuria or that the leading clique in Manchuria need to pledge support to this new government.
The main problem here is, that a weak Warlord China could encourage Japan to conquer or vassalize parts of China.