They aren't avoiding the Sinister Six. They are planning it for movie 3 or 4. They even have a studio lot called Sinister Six.
Just because Harry is a Goblin doesn't mean he is the Green Goblin. They seem to be doing what the did with SM3 and just putting him in a costume. Norman will still likely be the one, true Green Goblin.
That's a myth. Ditko was planning Osborn from the beginning. In fact he's a random background character as far back as issue 25.
http://forums.comicbookresources.com...1#post11913537
http://www.supermegamonkey.net/chron...rmanOsborn.jpg
Even if Norman had been a stranger, it wouldn't have created that theme. It would have been done away with the moment the Goblin came back, especially if we found out who he really was (if we only had a face but not a name, it still wouldn't have made that theme. It would have just made a new mystery). Either the Goblin would have become steadily less important (because he's a random nobody) or the other identity would have been fleshed out (so the "faceless and impersonal" part would have gradually been lost). Besides, Norman and Peter never really knew each other, so while Norman wasn't a stranger he wasn't that far from it. Nor to mention that nearly every other enemy Spidey has faced really has been a stranger to him, and it would have been strange if he expected the Goblin to be somebody Peter Parker knew (the real theme is, evil wears many faces, and you don't always know who people really are).
The Norman / Harry / Peter dynamic is wonderful and makes Spidey and the Goblin one of the most complex, warped and interesting arch-enemy pair ups, especially later when Norman starts to see Peter as a more worthy heir than his own son and, following that, when he sent Peter a letter thanking him for saving him from being just another boring businessman, and suitably twisted stuff like that. Its a real pity that Norman no longer remembers that Spiderman and Peter Parker are one in the same.