Jen considers Tracey Davis to be her best friend, yet evidence shows that this honor belongs to Luna. Then, we hear about Jen and Tracey's miserable childhoods. While Jen had it worse, the best friend aspect makes more sense. She sees herself in Tracey and wants to give Tracey the friend and guidance Jen wished she had prior to meeting Elsie.
Dumbledore's outright Stupid Good persona might be grating until Book 3 reveals how Fawkes is a fragment of a Stupid Good entity that even the other Powers found irritating by the name of Enoch the Unending Wheel. It might be possible that Dumbledore's mind is being affected by this presence negatively after so many years of contact.
Similarly, it's noted that Enoch was specifically the opposite of Baron Samedi, Jen's patron Power. It's not improbable that on some subconscious level this is exacerbating Dumbledore's fear and hatred of Jen.
The prophecy specifies that Jen would be the twin "knowing only darkness." This was actually referring to her being blind. But, it also comes two-fold in that she is also a Dark witch and knew only hatred, which can also be considered a form of "darkness."
Sirius's rather abrupt turn from the side of Light back towards Darkness may be simply enough explained. Sirius turned away from the Darkness in the first place because of the positive influence of the Potters, especially James and Lily. In canon, they died and he used the very fact that he never betrayed them as his anchor of sanity in Azkaban, thus preserving his allegiance towards them and what they represented. But in this continuity they betrayed him, believing the worst of him and letting him be sent to Azakaban. And that very injustice, the fact that they turned against him even though he had done nothing, was what let him keep his mind. No wonder he wants nothing to do with the Light.
Candyland is Nightmare Fuel already, but Jen was not the first child prostitute there. How long had that place been running for by the time the series began?!
Jen notes after murdering the Dursleys that none of the neighbors even seemed to care that she was obviously being abused by them. Dumbledore is later revealed to have been altering memories of students after the events of the Chamber and Troll incident. Did he have anything to do with the neighbors' seemingly apathetic behavior?
Jen removes the charm Luna placed on her bed to prevent herself from feeling sexual desire. We don't know if Luna was aware of the charm being removed or if Jen told her. If that is the case, then it is possible that her relationship with Jen was reaching some kind of Questionable Consent, since Luna wasn't aware of the entire picture.
