Russia would *always* have been involved, simply because of the differences in political system. The Nazis were actively opposing the spread of socialism/communism, and this was one of the cornerstones of their idealogy. Had Hitler defeated the UK (which he nearly did), he would have invaded Russia anyway. He needed the "lebensraum", and had no reasons not to attack.
As for Russia being able to attack Germany: in '41, they weren't. In '42, they might have kicked some arse, even without the experience defeat gave them. Stalin was actively trying to improve his military, especially after he saw how stupid his purges were. This can be illustrated by the T-34 tank, which was vastly superior to anything the Germans had at the time.
The reason the Russian army was almost destroyed is simple: they weren't able to communicate properly during the blitzkrieg (which is one of the cornerstones of that tactic!), when an army *needs* to communicate in order to react to an invasion. Also, Russia's army had been told *not* to fight the Germans, or to be more specific: they had been told not to react to German "war games" prior to the invasion, for fear of prompting such an invasion...
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