What happened with Stu & Jade starting a relationship at the end of the first film? Simple, he mentions at the start of the sequel that he's still seeing a therapist to cope with the events of the first film. Stu simply couldn't date Jade because she & Tyler were reminders of what happened in Vegas.
Phil initially comes off an obnoxious frat boy, in contrast to Stu as the responsible one. However, once the reality of losing Doug sets in, Stu basically has a breakdown & Phil takes control of the situation whilst calming down a distraught Alan. Phil's a married teacher with a son of his own, so it makes sense that he's the one who becomes the voice of reason rather than Stu.
Doug was severely burned after the guys found him, to the point of his chest hurting like crazy when somebody touches it. Then in the second movie, from the prostitute's story, we can tell that Stu, as Alan puts it, "has semen in him". Additionally, he's been treated with a needle from a not-so-reliable tattoo artist. Now, Stu is a responsible and caring guy, when he's not stoned. It's safe to assume that he'd take all kinds of tests before endangering his new wife. This means that neither of the grooms had sex on their wedding night. And you thought that (missing fingers aside) there had been no consequences.
In Mike Tyson's first scene, he's singing to the song "In the Air Tonight" by Phil Collins; after the drum solo, he tricks the guys into singing along until he suddenly knocks out Alan. The song itself is about an angry man who hides his rage towards his cheating spouse and the drum solo is the moment where the man finally loses his temper. Pretty fitting since Tyson was infuriated by the abduction of his tiger and made the protagonists drop their guard when he convinced them to sing along. Like the song; Tyson was hiding his rage until he unleashed it at full force.
In the first movie, when Stu is freaking out over Alan wanting the "free" almonds in the room, Phil picks up the almonds, gauges the weight, and then replaces them with a glass of water of equal volume, thus tricking the pressure-sensitive plate to not charge Stu. Brilliant once you remember he's a science teacher.
As mentioned in the Fridge Brilliance section above, in the second film Stu has sex with a prostitute and gets a tattoo with a potentially dirty needle. This not only put his own health at risk, but carries the risk of infecting his new bride with an STD.
Stu was high out of his mind when he had sex with the prostitute, meaning that he could not give informed consent. The prostitute raped him.
