I don’t see a benefit for leaving any troops in Amur. That is right on Japan’s front door and it is easy for Japan to deal with them when it is convenient. It is still suicide for the Russian troops - just suicide on a smaller scale.
I don’t see that it offers much benefit in terms of Russia being able to assist China either. If Japan wants China, she will get it. The job of China is to die as slowly as possible. And Russia is under pressure from Germany so they typically don’t have the units to spare in assisting China.