This is really interesting I hadn't thought of it as intentional! But perhaps it was! Executed poorly of course, it was Safran after all but that is a really interesting way to look at it!
How they acted was definitely s reaction to dans book at least - Chuck got so much development and progression as a result of the realisation that he didn't want to 'die alone in a closet'. Serena was so shell shocked by her degradation (as she should be!!!) that she became even more lost as a result. She tries to find herself at the Spectator and Nate fires her. She's so isolated by Dan and Blair that she is pushed to turn to bloody gossip girl as a therapeutic release, which was never going to end well. I always think- it's just so weird that Dan tore her to shreds so badly.. Are we supposed to forget how much he loves her?? It's a Shame that in order to place Dan and Blair together they had to completely ruin Derena, by creating so much indifference in Dan, that I never bought as organic. Blair - well Blair was a complete mess after the accident, and you're right about Dan, he just doesn't possess an ounce of self-awareness, which to me is so unappealing as a character trait, since he judges everyone else. I didn't care in season 1 because he loved Serena so much, he genuinely wanted the best for her as well, so his judginess was just a vice, that all characters had. The rest of them all had flaws, of course they do, but He seemed to think he was morally superior and that was hard to stomach. But you're right, if they had handled this differently, it could have been been a lot better than it was anyway!

Was anyone happy about lily ending up with William by the way?? I don't like how he treated his daughters and he got away with so much!! That always bothered me