Bee corrupting Melanie makes perfect sense, as it was Melanie's interference that derailed her plan in the first movie by motivating Cole to stay awake. She saves Cole and gets him together with Phoebe, but also gets revenge on Melanie at the same time.
Melanie's Face–Heel Turn to become an Instagram celebrity makes sense with the revelation that she lives with Parental Neglect, dealing with a drug-addled father who cares more about his Cool Car than her. Of course she would desperately seek the approval of others.
Cole's parents trying to ship him off to psychriatric school is quite harsh. But seeing that Cole had driven a car into the house, and he's being severely bullied at his school, it might have been in his best interest.
During the “Apache (Jump On It)” dance fantasy sequence, the Cult members are each shown making a face of shock and horror at the camera while Cole and Phoebe continue dancing with each other. Given that this sequence is a metaphor for Phoebe and Cole losing their virginities to each other, those dismayed reactions of the Cult are probably to represent how this coupling will taint Cole’s blood and doom the Cult.
Max is the exception, having a neutral expression. He is later shown to be the least mad about Cole and Phoebe tainting their blood, downright impressed that Cole got with a girl.
Their confused reactions could also be them being Squicked out at the two having sex, due to both being underage.
Melanie referring to her and Cole as "Thelma and Louise" rather than the more iconic "Bonnie and Clyde". In Thelma and Louise, the two women kiss (presumably platonically) before driving themselves off a cliff. Melanie was planning on kissing Cole, possibly to further manipulate him, and then betraying him (though she was unlikely going to commit suicide as well).
Also, Thelma and Louise were friends. Bonnie and Clyde were lovers. It clearly shows that Melanie didn't see Cole as a love interest, which makes the selection of Thelma and Louise even more appropriate.
At the start, Max says that if they are defeated, they can still wait another two years to revive and try to kill Cole again. That either means they can return for a third film in the franchise, or worse, they can be revived indefinitely until the day Cole dies. Granted, Cole and Phoebe have tainted their blood, but if there's a loophole or the cultists just want revenge, nothing is stopping them from coming back to haunt him.
The movie's ending is actually quite bittersweet, despite the optimistic tone. Cole survived, defeated the cult once again, his father now believes him and he's got a hot girlfriend. However, not having proven the cult's existence to anyone else aside from his dad and Phoebe, he has to pretend he was acting out and nothing happened. Not to mention his best friend betraying him and dying brutally in front of him, which he will all have to keep to himself (and Phoebe and his father). It can be seen that he's becoming a Stepford Smiler.
