I personally do not see how they've "ruined" the Dan/Blair arc. I mean, technically it hasn't really begun. It's just revving up if you ask me. It's a much slower burn but it will progress I'm sure. To date there is no evidence that indicted Blair/Dan won't have meaningful interaction in the last two episodes. That the promos are misleading is a given so I wouldn't base the entire arc by one or two failed attempts to create Blair/Chuck drama. It is quite in character for both of the to tread super lightly when it come to the other (talking about DB here) and so I think the writers are right on track with how, if this were RL, the pace and nuances of the relationship would go.

Throwing the Prince in there is certainly a twist but I believe he's there to show Blair that happily ever after doesn't come swooping in with a Vivier shoe and a title. She needed to know that to even begin to see Dan as a leading man in her life.

Again, I'm convinced the finale will indicate Dan/Blair are on a path to togetherness.

So, still not watching the episodes until I have confirmation that there will be DB scenes. Then I'll just yt those, because I refuse to lay my eyes on what the show's aftermath of the "no it's not abuse it's love!' SL would be, considering the chair on chairs photo and all the other promos featuring Chuck with other characters who are not beating him to a pulp.

ETA: Before I thought I would have been ok with a 'friends' ending between Chuck and Blair. Thought it would be nice for them to have closure, and close that door forever, and it'll be good for both of them to finally move on. But now, I feel my skin crawl even just looking at a promo shot where Chuck is standing close to Blair, or looking at her, or touching her, or interacting with her in any kind of way. I just want him to stay the hell away from her forever.