The narrator in this story is a bit of a crazed, jealous, spiteful man who ends up bricking his friend behind a wall in order to ensure his death, and, he does it all with panache and enjoyment. So, I wouldn't rely too heavily on him-he's not necessarily trustworthy. The story is told from his point of view, so it is first-person, and, because of the things I listed above, he isn't a reliable narrator. He might be relied on to have told the events of the actual story accurately as he can, but before the story occurs, we don't really get an objective picture. For example, why is he so angry with Fortunado, and did poor Fortunado really deserve such cruelty? The only clues that we get to these questions are Montresor's words himself. He states,
"THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge."
So, we can't say with any certainty that the narrator's reasons for revenge are justified. We don't know the details, and can't make a judgment call for ourselves. Hence, the narrator is a bit unreliable, since his entire mission is based around something that he deems worthy of revenge, without telling us what it is. I hope that those thoughts help; for your other questions, I suggest submitting them separately, since the format of the website allows one question a day. Good luck!