Here is what we know:
Dave was on the verge of releasing Blair's chapter - which we KNOW is negative. So clearly when he went to see her at the end of 6.04 it was NOT to get her back.

Then he sees the engagement ring and learns just how serious Chuck and Blair are and decides to nix the Blair chapter and release a POSITIVE one on Chuck.
Per someone who watched this it's part of Dave's "seduction" of Blair.

RE: The Good Chapter on Chuck
Why would that make Blair want Dave? It's not like Blair thinks Dave is a bad guy and reading the positive chapter he wrote on Chuck makes her reassess Dave as a person. She already thinks Dave is swell.
Also it's not like the positive chapter on Chuck would make Blair suddenly reassess Chuck and want HIM. She ALREADY wants Chuck.
I think we are missing what Dave is trying to do with this positive chapter on Chuck.

RE: Dave's change of heart
To me that suggests Dave just wants to win against Chuck. He wasn't trying to get Blair back until he saw that engagement ring. The ring would not give him hope that he had more of a shot with her. It would have the OPPOSITE effect.
I think the bottom line is he's desperate for Chuck Bass to NOT win the girl.

Dorota and Dan talked about the ring.

Dan triy to play the good guy because he wants to be with B again,

He wrote a good article about Chuck (he say in it that Chuck is a good guy who fight against his bad father) and he ask Blair to go with him at the cotillon.

He really tried to look like the nice guy.

Steven wants to ask Serena to marry him and he ask Lily her blessing.

But Blair and Sage want to mess his proposal.

Blair actually only want to promote W desighn so she asked Sage to wear one of her dress.

Blair called Lily and she tell her that S is not ready.

At the cotillon ,G told to Dan that he’s delusional if he thinks he can be with B again

S and B have a fight again about B’s scheming against S .But B explained that she only wanted Sage to wear one of her creations but she didn’t want to hurt her and she wants to be her bride maid. They even hug.

But during the presentation of the girls, Sage sent to Gossip girl the sex tape.

D and B have a fight and B said that she was devasted when Chuck slept with Jenny but with him it’s worse because she thought he’s better than Chuck

Blair and S have again a fight. B said S was right, their friendship is over.

Then, chuck and Blair are in empire. But there is no kiss. Lily didn’t want to help chuck, so B still has to wait .They talked about the sex tap and she thanks him for not having sleep with S.

Chuck talked about Dan. He always thought than Dan was better than him that’s why he was afraid.

Ivy and Chuck tried to manipulate Bart but they failed.

Merci @chairaddict68 who watched & shared the summary with us.