1) How did Akira survive? Unexplained but Natsuno does a lot of things that they don't spoon feed you what happened. Such as hypnotising the head guy to kill werewolves. So Natsuno was probably still looking out for them 2.
2) Who bit Toshio the first time and why wasn't he phased when Chizuru tried to bite him again? (episode 18) The other biter is clearly Natsuno who hypnotises him the opposite way, my guess is Natsuno bit him afterwards to counter the hypnotism.
3) Why was Toshio chopping the tree? (last episode) The were trying to chop the trees down to stop the whole village being burnt down.
4) How did Yuuki find out that his son is still "alive"? Not sure what you mean by this one, but we do see that Natsuno is back at his family home with his father who has gone loco.