Hm. Okay. There are a lot of points to tackle in reply to this.

A. I think I'll start with the easiest one: your claim that Rowling's teenagers aren't believable. As a teenager myself (I'm nineteen) and as one who has been a teenager for most of the (if not the entire) time that Harry, Ron, and Hermione have been teenagers, I find them and the other students to be very believable youths. Their struggle with growing up is, to me--again, a teenager--completely realistic; they laugh, fight, and play like real teenagers do, and yet, when called upon in truly dangerous and dire situations to be adults, they rise to the occasion in the way teenagers would--mostly with success, but with the occasional teenager-ish outburst (Cf: Ron's leaving).
Could you perhaps give me a few specific instances of moments where you felt that Rowling's teenagers were unbelievable?
B. Secondly, you state that you were "appalled by Rowling's disgusting views on the meaning of life and death and the essence of magic". First, I would like to ask what exactly you find disgusting about it, and what you consider her views, as evidenced by the text, to be. Without really understanding what it is that so disgusts you, I can't fully take on that point.
However, I myself see nothing "appalling" or "disgusting" in Rowling's presentation of life, death, and magic; I rather see it as a beautiful presentation of these ideas. In fact, again, I would argue that her views on the matter are much like Tolkien's--they are/were both Christians, after all. But until I know specifically what it is that bothers you, I won't treat on this in any more detail.
C. Okay, next, I guess, your complaint that you thought Rowling's writing became "vague and nebulous" in the later books. Simply based on my reading of the novels I would have to fully disagree; I literally have no idea where you're coming from. I find all of Rowling's writing to be fully directed towards the telos of the entire work. Again, however, if you could elaborate on what you mean and perhaps give a few examples I would be better able to answer you.
D. You state about Harry, Ron, and Hermione in
The Deathly Hallows that: "They spend three hundred and something pages wandering around, screaming at each other and wondering what to do about them [H]orcruxes." On a very literal level, yes, this is true.
However, on a deeper level, these three hundred pages or so are about so much more than simply wandering about the U.K. Firstly, as they wander around, the plot advances as the trio searches for the Horcruxes and slowly finds them, one by one. But this lengthy wandering also serves as an illustration of the desperation of the situation in the Wizarding world, the horror that will be, in an even more realized form, if Harry is unable to defeat Voldemort in the end.
E. Similarly, the countless deaths of "a whole bunch of irrelevant characters" serve to highlight that Voldemort is a terrible threat to the world's peace and stability; hundreds, perhaps thousands, of people are dying due to Voldemort, and he is not yet fully in power. The situation is clearly utterly dire. Just as, in
The Lord of the Rings, the Scouring of the Shire was necessary to the book to show the great scope of Sauron's evil, to illustrate how absolutely
devastating his dominion would have been, so the numerous deaths in
Harry Potter are necessary to demonstrate the great danger that Voldemort poses.
Additionally, you claim that these characters die "because Rowling wants to say something about death." I don't really think this is the reason that they die; I think that it is merely, as stated above, necessary that they die to show the great scope and magnitude of the violence wrought by Lord Voldemort (just as, again, the Scouring of the Shire is necessary to
The Lord of the Rings).
F. You also say, in relation to the 300 pages of wandering: "Ron leaves, we don't care because we know he'll be back, Harry has some convenient dreams of Voldemort, they get attacked by a giant snake, and basically everything else is just filling." In the interest of time (and more important points), I'm not really going to argue with this, I'd just like to say that I don't feel like anything is filling, and I think it's all terribly important to the story, and I would also like to ask you a question (in response to the first statement about Ron leaving):
When you read
The Lord of the Rings, surely you knew that, because it was a novel, it would end happily--with Tolkien's great eucatastrophe--but did that mean that you did not
care that it did, in fact, end well? Though you
knew that this, a story about a great battle between good and evil, must necessarily end with good triumphing over evil, with the Ring being destroyed in the fires of Mount Doom, were you not nevertheless filled with joy when Barad Dur came crashing down and Sauron was vanquished?
Just because you
know that something is going to happen in a novel doesn't mean that you can't appreciate it or care about it or that it doesn't matter.
G. "Harry's parents cheerfully encourage him to walk blindly to his death, while Malfoy's parents endanger their lives and their entire mission just to find out if Malfoy's alive. Harry sets himself up as a Christ figure (this is where I as the reader start to feel really uncomfortable) and Dumbledore gives us another plot dump despite being dead, then, unsatisfied, Harry spoils the moment of Voldemort's death with another plot dump, that really pi$$es me off because he basically says that the sacrifices of the brave wizards dying to defend Hogwarts don't count compared to him going up and standing there while Voldemort kills him, which he survives anyway, and that his sacrifice would ulimately defeat Voldemort. I find that completely morally wrong."
In relation to this passage of your argument, I'll mostly treat on the statement that Hary "basically say that the sacrifices of the brave wizards dying to defend Hogwarts don't count," but what I'll have to say about that relates to the rest of your complaints, except, perhaps, the one about Harry as a "Christ figure," which I'll address now.
It is true, Harry can be seen as a Christ figure. If you are not Christian, and, I suppose, especially if you are atheist, I can see why this might bother you. However, I think the tie-ins with Christ can be easily separated from the story itself, and I would argue that this sacrifice of Harry's is necessary to the story. (And, by the way, Voldemort didn't actually "kill" Harry; Harry never died, but rather went to a sort of Limbo in which he was able to communicate with someone who
was dead--Dumbledore.)
As for the rest: Harry does
not say the deaths of the "brave wizards" do not count, that their sacrifices do not matter. He does not in any way imply this; he does not belittle their deaths or their courage, but rather honors it--it is for this that he fights, it is for this that he is willing to die, for this that he thinks he
will die; Harry is willing to die, to face Voldemort alone, so that no one else
need die. This does not mean that he scorns the sacrifices of those already dead but that he is pained by the fact that their sacrifices were necessary and seeks to save others from having to make the same kind of sacrifice.
In short (and this is where this relates to your first objections), I think that you have completely failed to understand
Harry Potter, that you fully misconstrue the point of it, the message behind it, and have failed to really comprehend what it is that Rowling drives at with the series.
H. Your complaint that Harry Potter is a "passive character." At this point, I'm really too tired and lazy to address it fully, but, rest assured, I WILL! (In a later post).

I understand, of course, if you really don't feel like responding to all of my questions and elaborating, etc. I won't be offended if you leave the conversation as it stands (though, again, I do intend to address your last point--probably next week, when I have more time).