It's interesting to read about all the theories about how VT could have been in Joanna's flat prior to her arrival at home, but the problem with these scenarios is that no evidence was presented in court to support the theory - not from the prosecution, not from the defence. The beauty of the theory is that it provides a neat package for explaining the reason behind the assault: he was prowling, got caught, attacked her, panicked, hid her body. This theory also seems to completely support the manslaughter charge. That is, it suggests that he was in the apartment for unknown reasons, was surprised by her when she arrived at home and the murder was spontaneous.
I don't think this murder falls into a neat package like that. I think there was something wrong with VT's head that night. My gut feeling is that he was in a rage for some reason, that he went to her door and attacked her. I keep coming back to Joran v.d. Sloot, whom I believe went into spontaneous, violent rages; attacked women and was then quite instantly again calm. That said, I think that VT has very cleverly wrapped his testimony around known facts in order to explain the evidence and that he has made a good argument for manslaughter. I see one of the main weaknesses in his defence being that he provided a new explanation during testimony - making it very obvious that he looked at the facts and concocted a story.