Velma being a good big sister to Madelyn, especially since at the time of this movie was released, Mystery Incorporated depicted Velma as being rather cynical, even to her own family. One could interpret it as growth and maturity on her part.
Velma telling off Shaggy for referring to Madelyn by her Embarrassing Nickname of "Doe Eyes Dinkley". Can be seen as Heartwarming in Hindsight, as Velma says that she doesn't want him to crush Madelyn's heart by making fun of her and it's rude in general — In Mystery Incorporated, Velma herself was already hurt by Shaggy's insensitivity, and learned from Marcie how hurtful mean labels could be, which is why she stops referring to her as "Hot Dog Water" in Season 2.
We find out that Curdles was trying to buy the castle so that he could give it to Alma and she would take him back after he left her for so long to make something out of himself. Seriously, an entire castle! But she loved him from the start anyway.