Sure, Megumi discovers the truth about Toji being his dad in the manga. It's a crucial plot point that brings about various changes in Megumi's storyline and his relationships with others.
I really don't know. There are so many manga out there. Yuki's choice could be anything from a fantasy epic to a slice-of-life story. Maybe you can look for manga-related items around Yuki to get a clue.
It depends on your tolerance for spooky things. Some people find any ghost story scary, while others might not be as affected. The way the stories are told, the atmosphere created, and the specific elements in the Yuki Ghost Stories all contribute to how scary they are.