The Time Machine affected the time stream a little as the characters arrived in 1986Explaining why Poison was popular before it had any chart toppers, etc.
The bear suit guy is also a time traveler.When they first get into the hot tub and start drinking they start to see a lot of crazy shit. What they are actually seeing is other unwitting time travelers like themselves flying through time. They bear guy is one of these people.
Jenny was always going to break up with Adam, but when he did it the first time, she couldn't take it, she got pissed and stabbed him in the eye with a fork.Also, the "It's Not You, It's Me" thing could have annoyed her.