Visa application/ proof of legal status and round trip tickets

Hi,
After my semester abroad in Moscow, I will go to China by Train/Transib train. I have already read which documents are required for the application. But I have still two further questions:

1) Proof of legal status in Russia: Is my student ID enough? Or which kind of legal proof is needed?
2) Round trip tickets: I will enter China from Mongolia by train. But the train tickets are not available yet. The ticket will be available during my tour to China. I already have all hotel bookings and the flight ticket back to Germany. Would that be enough?