Why does a now overweight sports fanatic still walk around in shorts wearing the same backpack he was captured with?
Shorts for hot weather and same old backpack because he's probably a student with a practical need for it - there's a college nearby that both Abbie and Sarah go to, no reason he couldn't also attend.
Where are the police, who will gladly arrest a bunch of people for playing a harmless game with dangerous looking props, but let the hunters get away with: attempted murder, possible murder, kidnapping, brainwashing, and extortion?
My guess is more that the geeks are out in public doing stuff during the day with people to report it. The Hunters are normally out at night and usually go after geeks that are in beast mode. So unless the beasts also get noticed the hunters probably won't be either, and if anyone did report it they'd sound nuts.
Solved...turns out, the hunter thing WAS NOT REAL!
If the hunter thing wasn't real, then how did Joel and Mark even meet in the first place?
Presumably Joel was doing the same thing he did with the vampire LARP - he just dragged Mark into it and acted like it was real, without explaining otherwise. That'd also explain why Sara got so angry with Joel when Mark mentions being abandoned to the Hunter by Joel. To her, it means that Joel threw Mark into two LARPS with no explanation, and abandoned him during one that seems to be much bigger and more elaborate.