Jo had absolutly nothing to do with the casting of the movies. She and Evanna had written back and forth before Evanna was ever cast as Luna, and she had told Evanna there were definitely similarities between her and Luna in the letters, but she did not promise her the role, she had no idea Evanna had been cast until the directors called her.
It is especially seen by mentioning Marauders and Harry having an insight in their time spent in Hogwarts. The point is; LIFE changes people; it is how these young people full of life and joy become damaged adults, it is about their loss and demons they are fighting. It shows that life doesn’t spare anyone and nobody knows what is ahead of him. Jo wanted to show that change, how certain events happen and effect people, leaving them destructed and damaged. In my opinion, that is the point of characters as Sirius and Lupin. Nobody is spared and everybody are living with their burdens.
honestly do not think Dumbledore is gay. JKR should have wrote it into the books if it was true. Her saying it after the series was completed kind of made it seem like a ploy to attract more readers.