However, in the game Russian HQs can field only 7 Corps (12 for main Army) while German can field 8 Corps. Also, from what I gather one cannot attack the same hex simultaneously with more than one Army at a time, am I wrong?
If it is the case, that also render the large amount of Russian armies useless as well, because even on one-to-one basis Germans can field more Corps than Russians in a single HQ.
So I am confused about my options.

How can the Russian player use its enormous number of Corps to smash the German and Austrian armies in battle, if each German army can field a bigger max of Corps individually than a Russian army? Because if the average Russian Corps is worse than the German one, the only hope for Russia is to send even way, way more Corps in battle on tinier German forces and hope to flip them and smash them through sheer numbers of Corps.