Why do you 'not believe Slytherin won the past seven years? There's literaally nothing to contradict or support this in the book except a statement. If it just seems unrealistic to you...well then that's that, I suppose. Slytherin did win the Quidditch Cup--not everything was taken away from them. As for Dumbledore playing favorites, you'll find this is addressed later.
Ah, I see. Looking at this moment in the context of the series as a whole, I'd say this sort of behavior is not in character for Dumbledore--he really does make a point of putting persenal feelings aside, unlike Snape, who really runs rampant most of the time--but I'll holdoff on a deeper explanation until you get to...let's say book three to avoid spoilers. Though I'll be interested to see your opinion by the end of book two, as we have a slightly tweaked situation, that isn't the subject of any debate amongst fans, unlike this one.
As someone who's reread book four recently, I'd have to say Harry's trauma still holds up. The delicacy with which Rowling handles it is really impressive, since as a kid I was kind of bored by the end of Goblet, feeling like it was just "he's back, the end", but now that I'm older I see how well set up and believable everything is. I was referring to the end of Chamber--Dumbledore gives Harry and Ron awards and cancels exams--it's a more low key thing to do than the points since the trophy room never seems to be visited for pleasure, and considering that Colin was unconscious for seven or eight months, it's much better to send the kid home with some work rather than have him stress. Plus, the entire school's been impacted by this, unlike with the stone which no one knew about, so why not give everyone a break to spend time with the people they were worried about or just breathe, cause everyone's ok and Quidditch is back and the school isn't going to be closed. I think you also see with Goblet, how little control Dumbledore has in the larger scheme of things because of the career he chose; he can give advice/his opinion, but a magical cup had more power to effect how things played out than he did. I think that one of the minor themes in the series, that adults are fallible, and we start to realize the older we get that they can't just magically fix things, is introduced very naturally in Goblet, which, along with everything I said before, makes me more inclined not to hold the end of year feast against Dumbledore in book one. I'll be very interested to see what you think of book 5; lots of revelations there, and the continuation of Harry's trauma combined with the fact that he's still growing up is, to me, very well handled.