Re: Happy to disagree :)
1) I don't think we know that creating a single horcrux changes your appearance. If it doesn't, then Dumbledore wouldn't have identified the change with the creation of horcruxes at all.
2) There could easily be other Dark Magic that causes the practitioner's appearance to change like that.
3) Not a problem on the same level, but if Dumbledore knew about Voldemort having a horcrux that early, we've *really* got to ask why Dumbledore didn't start looking for horcruxes earlier than he did. This was loooong before the prophecy suggested how things would play out. I'd excuse him for not going after horcruxes right after Voldemort's interview, because he apparently hadn't done much yet, but Idon't think Dumbledore would have had *any* excuse for not looking at least as soon as the war started.
Knowing about the Harrycrux from the start makes Dumbledore look bad in some ways, but at least he seems less incompetent.
Re: Dumbledore's views of Divination: I *think* there's speculation that the whole thing, including his decision to hire Trelawney, was a ploy of some kind. At least until the prophecy started to come true. The fact that he let Severus leave with part of the prophecy, rather than Obliviating him, has been used as one reason to believe this. I'm not especially familiar with the theories surrounding that part of the books, though, so here I'll defer to people who are.