She already had feelings for Brandon way before she and Dylan ever broke up. I get that she seemed off after the Dylan sighting, but think it was because she felt guilty - after all, unlike Dylan, she knew that she had had feelings for Brandon well before she and Dylan broke it off. To me, "We are so over" read like something she'd say to herself in anger, like "Don't need to feel guilty, that idiot does not deserve my worrying."
I totally see your point, and agree that she was already harboring feelings for Brandon pre-Dylan break up. Not to get started on another rant, but... :) That is yet one more thing that drives me crazy from Season 4. Post-retreat, she seems firmly Dylan despite kissing Brandon. Then when they want to move Dylan more into Brenda's orbit, Kelly is suddenly giving Brandon the eye over David's piano playing. By the time Kelly and Brandon meet in the parking lot and she does her "peachy weachy" baby talk, I think you can tell she's looking for an excuse to get rid of Dylan. She's saying things to Brenda like you're the love of Dylan's life and telling Andrea it wasn't meant to last with Dylan. Okay, fine. As a Kelly/Dylan fan, it's a bitter pill but I can swallow it. :( But then, Season 5 rolls around and Kelly clearly still feels a pull to Dylan and Dylan definitely still feels one to Kelly. They bring up the soulmate thing and then add the past lives connection. So, what was the point of trashing them during Season 4? Why couldn't they have grown apart organically out of their different views on college or the skeeviness of Kevin? I think they sped up their expiration date to get to a Brenda/Dylan and Kelly/Brandon hook up. They made it so ugly, just to repair the damage a few episodes later when Kelly and Dylan reconnect when he's in rehab. Again, this is why I tend to skip these episodes. My poor blood pressure! :(
Anyway, back to my point (I think I still have one :)), I don't think it was just guilt that motivated her. I think her telling Brandon that she wanted to be alone and needed space, then sneaking over to Dylan's at 2:00 am does speak to motives that are a little shady. Her response was for Dylan to get dressed and they'd go out for coffee before she saw the other woman, so I think she wanted to talk to him not just apologize for her lack of consideration. She knew he had been drinking before he found out that K/B were together, so I don't think she felt guilty for his relapse. I think she wanted to help him because she still cared about him, and part of her wanted to see if there was still something between them. I do agree with you that her "We are so over," was motivated by her own anger at herself for worrying about him. I just think it was tinged with anger for thinking that she might go back to him and feeling foolish when she saw what he was up to. I also think her "Are we even?" comment was telling. If she was over Dylan and completely in love with Brandon, why would she be hurt by him kissing some stranger? It should have been no biggie, like when Steve kissed Celeste in front of her.
I do think once Dylan went into a coma and rehab, both Kelly and Brandon were better friends to him. It's the part that came before it when he was falling apart that I don't like their attitude. If you see someone you care about hurting, you don't use it to put him down. I haven't seen the intervention episode in a while, but I don't think I cared for them in that either (although I loved Andrea mentioning him not visiting Hannah). After they realized how bad off he was when he nearly died, I think they improved remarkably.
I do think Dylan was a high maintainence kind of friend, and can see how Kelly and Brandon would find it tiring especially during a phase in their lives when everything was going great. But I think Dylan was a good friend to them when they needed it (Brandon's drunk driving, falling off the cliff, U4EA fallout, Kelly's near rape, her dad letting her down, her eating disorder and image problems), and I think it was a shame that they didn't return the favor with the same thoughtfulness Dylan had offered his help with.
The hostile and selfish way he treated Kelly towards the end of S4 was enough for her never to speak to him again, but he just returned from the summer assuming she'd be there for him.
I definitely agree that Dylan was a selfish jerk towards the end. I do think Kelly should have explained what was bothering her about Kevin and Suzanne, though. She was very passive aggressive with him. I think he thought she was jealous because it came across that way, when I don't think that was her actual problem. I don't think she liked the changes it was creating in Dylan, but she never articulated that and actually blamed Brenda for problems she wasn't causing. (I do think Brenda was coming on to Dylan in the kitchen scene and was ticked when Dylan didn't take the bait, but Kelly didn't know that! :)) By the time Dylan came back to town, Kelly and Dylan had broken up for the third time since high school ended. I don't think he saw it as permanent, but as a break because that had been their pattern for a while: fight, break up, get back together, make it work for a while, fight, etc. It's like when Brandon says that Kelly and Dylan were always fighting, Dylan corrects him that it was always complicated. ;) I think Kelly saw it like Brandon (at least at that point in time), that they didn't work out and couldn't get along while together, but Dylan saw it as a small piece of a bigger picture.
And taking Dylan out of it, I'm just irritated with Kelly in general. Those snotty faces that she was making at the Martins' and the comments about the debutante (have no idea how to spell it) lifestyle were driving me freaking crazy! How ironic that she refers to it as some kind of cult! Donna's not the one who has trouble getting out of cults. Better a Texas dip than a miledding! ;)