A few things:
What is the one thing that Harry desires more than anything? A family. A security net of people, which for the longest time, he never had himself, and in which he finds it with the Weasleys, and continues to build. It's something he's always seeking.
This is an excellent point.
What do you think an 8 would do in that situation? I'd want to say they'd seek vengeance for the murder of their family. But, that would be the makings of a villain. 8s can be heroes too, right?

I'd say a heroic 8 would still cry when looking in the mirror and seeing their dead family members who were unjustly killed. However, the next point is good: that his answer to this is to seek a security net of people. I do wonder, though, if a kid is that young, and finds a family that will accept him, if he'd go with it. For instance, I know someone who I'm quite sure is an 8 - he's classic - and stayed with my family for a while when he was young because his own family was not working out so well. He seeks autonomy, travels on his own all the time, won't settle down, etc, and always had a very independent/ autonomous/ anti-family unit mindset, but he still enjoyed the comfort and love of a new family. The Weasley issue doesn't necessarily seem incongruent. In that situation, who wouldn't feel better having a family who loves them? What, in the makeup of an 8, would cause them to necessarily reject that? I wanted to move out of my house when I was 16, and ran away all the time as a kid, but I knew I could come back. If I had no place to go at the tender age of 11, and existed in a world of magic where my survival was at stake, I can't guarantee I wouldn't take up an offer from a kind , generous family to take care of me, even if I wouldn't look to them for opinions on what I should do with my life, etc. It's a matter of survival really. (Although, if he's an 8, it does make a lot less sense that he didn't run away from his aunt and uncle when he was young.)
However, this point is an irrefutable one:
He's seeking guidance from authority figures, or maybe trying to replace parental figures, throughout all the 7 books - Mr. and Mrs. Weasley, Dumbledore, Lupin, Sirius. I don't see the go-it-alone, I-don't-need-anybody mentality in Harry. In the end of book 7, he does "go-it-alone" to face Voldemort, but the reasons why he does that are coming from a different place than a need for autonomy and control.
You're right here. He did it alone because he didn't want any of his friends to die because of him. However the other list of authorities that you mention here, who he leaned on for support (outside of simply, survival) is definitely more authorities than an 8 would bother with.
Making an alliance with Dumbledore is intelligent: it's a matter of having power in the school. If you have the most powerful person in your back pocket you have more power. Bingo. Some people would say this is 3-like thinking, but I think if Corporate CEO is a stereotype for 8s, too, then 8s must also know how to climb up the ladder. I'd agree, though, that Harry's particular method of suckuppery is a tad un-iddish, but it still *could* fall into the category of survival mechanisms, or gaining power, as a behavior (but you're right to point out, the motives are probably different). Living with the Weasleys could be seen as an act of seeking survival opportunities. However Lupin, Sirius, etc; unnecessary. Still, having allies does give a person more power. But the reasons are what we'd be looking at, in terms of enneagram. I can buy your argument that he did this because he is seeking a new 'family structure.' It does make more sense, with his character, than doing it because it's pragmatic. He gets very attached to these people and their place in his life. He goes to them for advice regularly.
I do think that Harry is CP6. One, his fear is not overtly present in his thought processes, the fact of the matter is he is still motivated by it. He fears losing the new life that he loves, Hogwarts and his friends. Also, Harry doubts and questions himself all the time, doubts his own mind, and exhibits circular 6-ish thinking. He does not think he's the hero that everyone says he is. This label that everyone thrusts on him never truly goes to his head. Two, he doesn't avoid his fears; he faces them head on, in true counterphobic fashion. Also, his rule-breaking and his questioning of authority that he doesn't agree with, which he sees as unfair, like with Snape. Harry does not blindly accept authority. And although 6s do not have a monopoly on loyalty, Harry is completely loyal to his friends, the Weasleys and Dumbledore. Granted, he wavers and questions that, especially when his expectations and illusions of Dumbledore are broken and challenged, but even then he's still loyal to Dumbledore...Dumbledore's man to the last.
Him not believing he's a hero is a key point you made here. I didn't think of that before when I posted (like I said, I didn't think it through, and it's been a while since I watched/ read the books) ... but you're right, the size of his ego is un-8ish. An 8 would probably feel right at home with that power, deep down, even if he didn't boast; but boasting would be inevitable as well, when he's less healthy. In fact, when I read the book, I remember thinking, "He's so badass, why is it so hard for him to think of himself as a hero?" Admirable modesty.
The circular thinking and doubting of his own mind - another good point, and something I'd forgotten from the book.
Can you pinpoint a disintegration to 3 and integration to 9?
His actions are also ruled by superego. He acts to make things right or to help people. He's courageous, but he's got a huge savior/white knight complex. (This is may indicate a 2-fix as well, although I haven't quite pinned this aspect of him.)
Savior/white knight complex - is this necessarily incongruent with an 8?
8s think of themselves as magnanimous. Usually more of a 'dark, independent' knight mentality, but still, any 8 will convince himself or herself that they save the innocent, etc.
World dominating 358s unite!!!
I can't help that I'm awesome.
*fingers kukri*
Oh god. I have one of you guys at home
Rofl! Lucky you
I hope you have some weapons of your own. =,)