The Germans did successfully invade Russia in WWI, sort of. They won, at least (even though it only happened with the help of the Revolution).
If the Germans press the Russians harder in the beginning of the war then the civil unrest in Russia happens sooner, and more violently.
Actually, if anything it doesn't happen at all. Even with the Tsar's fumbling, it took three years of constant screw up to bring about the Revolution.
If Gemany pours everything in Russia (wcith at the time included much of Poland) it is going to run out of gas. IOTL it never got TO Russia proper.
While it is throwing everything as the Russians, what is France going to do (Russia's sworn Ally aganst the Germans and a country that had a burning desire to kick Germany in the nuts)? It WON'T do what it did in WW II by making a demonstration attack and backing out. It WILL be in the Ruhr inside of 10 days.