I don't agree that Dan's shock, in the second episode, at Serena's sleeping with Nate is inconsistent with him being Gossip Girl. I don't think even GG knew about Nate and Serena until the story came out in the undignified scene at Bart's brunch. It's true that in the pilot GG said something like "Just when B and S had built a bridge, it all had to come crashing down." But as far as I remember, there's nothing to show GG knew why the bridge had come down.
As to the second point, I think that Dan, not for the last time, forgot the plan and followed his emotions. Afterwards he was sorry, of course. It's reasonable to think that in the third episode he was looking for a face-saving way to get back with Serena. In fact, that's what I did think even before I knew he was Gossip Girl.
I don't agree that Dan's shock, in the second episode, at Serena's sleeping with Nate is inconsistent with him being Gossip Girl. I don't think even GG knew about Nate and Serena until the story came out in the undignified scene at Bart's brunch. It's true that in the pilot GG said something like "Just when B and S had built a bridge, it all had to come crashing down." But as far as I remember, there's nothing to show GG knew why the bridge had come down.
As to the second point, I think that Dan, not for the last time, forgot the plan and followed his emotions. Afterwards he was sorry, of course. It's reasonable to think that in the third episode he was looking for a face-saving way to get back with Serena. In fact, that's what I did think even before I knew he was Gossip Girl.