First off, the scene with the sisters is not even close to manslaughter, certainly a colossally dickish thing to do, but embarrassing someone and then them deciding to run off into the forest only to die to a completely unrelated set of circumstances does not count as manslaughter, there isn't a court in the world that would convict them for that. It's not even negligent as they attempted to look for them after both Hannah and Beth didn't come back, likely under the assumption that she would go a little ways into the woods and cry for awhile before returning, there was no way to reasonably assume that they would run into a monster and fall off a cliff. The people responsible for the prank are massively insensitive, like many teenagers are, but they didn't commit any crimes.
Some of them may have been culpable enough to lose a civil suit and have to pay out money to Hannah and Beth's family for emotional damages, but even that would be a long hard case to prove, playing a cruel prank isn't a crime, and them running off into the forest would not be assumed to be incredibly dangerous in most realistic scenarios.
What Josh did was a whole hell of a lot worse than what they did to Hannah, I mean Josh would be fully justified in never talking to the assholes that pulled the prank, but what he did was in fact criminal, and broke numerous laws, especially the kidnapping, assault, and the whole choose who to shoot scene would rack up at least a couple of felonies.
Yeah, Josh is kind of tragic in that he was someone that obviously needed help, but most of the other people there did not deserve any of the sick shit Josh did to them, he's not responsible for the Wendigo stuff, as all that was unexpected on his part, but he responded to a cruel prank that resulted in a tragic accident by kidnapping and assaulting the people responsible. Even the people that weren't involved at all, so basically that kills most of his sympathy right there.
So yeah, every court would convict him, even if the prankers were guilty of a crime, considering he drew an entirely innocent person into his scheme with involving Matt (EDIT:I meant Chris, who wasn't in the room when the prank happened, sorry got them confused) . Also, kidnapping is still a crime, even if the original group were guilty of something, the court system doesn't let you just kidnap people for a crime they may have committed a year ago. Even if they were wanted by the police, you still get in trouble if you torment someone you've put under citizen's arrest as you are responsible for everything that happens to them once you restrain them.
I will agree that the Wendigo hunter should have lasted longer than he did.