[OK, as someone who has never played let me answer the question.]
How long are Starcraft II games now, after the growing pains are presumably over, as compared to Starcraft Original? [As long as it takes. If you're asking how long they're intended to be, ask Blizzard. If you're asking how long they are on average, either Blizzard or no one has that information.]
Is there an ebb and a flow, where one side can still make it back after a decisive loss in a single battle? [Yes.]
How decisive is micro in a battle? [Depends.] Are there any documented cases of "hero marines" a la Boxer win…