Early in the series, we see Dan yelling at his grandmother on the phone as well as "some explicit letters made out to someone named 'Grandma'". Later in "Traffic", while Dan is stuck in traffic, he states out loud "Why did that funeral have to take so long!". Later in "The Beach", when Dan is thought to have been stranded and will die, he points up and shouts, "Well here I come, Grandma! You jerk!" Yes, the writers are even more evil than we thought. "The Gym" takes this one step further, the implication being that Dan may have actually had a hand in her death. Only after stealing her deviled egg recipe on her death bed.
Since the cultists probably weren't all killed in "New Mexico", so they could have went back and continued the Human Sacrifice.
Also, probably a hand full of girls were killed in previous festivals.
The first part is unlikely as they were making a sacrifice for a good balloon festival. Since the balloons were destroyed, why would they continue the sacrifice?
In "High School Reunion", Dan learns he has the approval of his former classmates (who he really seemed to hate until then). Are his complicated (and dangerous) revenge schemes merely the result of years of experience, or has Dan been going insane since high school and nobody noticed because they thought he was cool???
Why would Dan think his classmates he hate him? Dan said his adolescence was "[...] constant ridicule and that was just high school." implying that his home-life wasn't too good either. If you take it a step further, whatever it was, it caused him to be wary of people and assume the worst which in turn made him assume every minor thing his classmates did was a sign they were out to make his life a living hell. I mean, would it be any different than he acts now? Or since they loved his revenge schemes so much, they could have been intentionally provoking him for their own amusement.
In "Summer Camp", we learn that Dan's Start of Darkness occurred when he was still a prepubescent boy at the latest. And that's assuming that his widely hinted at Hilariously Abusive Childhood didn't have him screwed up since infancy.
The last time Dan saw his camp counselor, a child untrained in archery was trying to shoot an apple on the top of his head and it is implied that the child may have missed and killed the counselor.
Implied, nothing. Chris states that the Warriors regularly play "William Tell" with the smaller kids, and the camp "loses" one or two kids to the game every summer.
Actually Chris is a Yetiman so that could be part of it.
Chris's being a bear-man (and his earlier ability to ride a grizzly) are part of his latent Canadian-ness, which he only expresses when he's in Canada. They don't come up in "The Family Camping Trip" because that happens in the US.
If the average human can WALK 2mph, how come it takes everyone too long to jog/run a marathon in "The Cat Burglar"?