Paul, my take on why everybody was suddenly in love is just the fact that they're coming of age. In the first book, the kids were only 11 years old, too young for most kids to be interested in the opposite sex yet. By this book, Harry is almost 17, though he started having "feelings" about Cho back when he was 14. So I don't think it's all that unusual. Now if they were all adults, I'd think pairing everyone up would be just a contrived wrapup to a series.
As for you theory about Dumbledore faking his own death, it's possible, but I think he's really dead just because the magic that places a portrait of previous, dead headmasters in the headmaster's office added a picture of a slumbering Dumbledore. Sure I think that Dumbledore could have faked it, but somehow I don't think so. You could be right, since we're all just guessing at this point.
I re-read Order of the Phoenix just to remind myself of what happened with all the references to book 5 in book 6. It seems that while I think Dumbledore's dead, it's possible that Sirius is not. We never got confirmation about exactly what spell he was hit by. Did Bellatrix actually hit him with an Avada Kedavra? Or was it another spell that stunned him? He fell behind the veil, so we're only guessing he's dead. He could very well be, but it is strange that JK Rowling wouldn't give us a body. I have a suspicion that the arch leading to the veil may make a reappearance in book 7 since it was never really explored in book 5.
