It is characteristic of Poe to begin his stories with expository-type introductions that read very much like very logical, dispassionate nonfiction. Poe had an analytical kind of mind. In "The Cask of Amontillado" Poe through his character Montresor discusses the whole subject of revenge in a rational, analytical manner before launching into a story which builds to a horrible ending. The introduction serves as a contrast to the story itself. Montresor meets Fortunato up in the streets where everybody is celebrating and having fun. He lures his victim deep underground and finally leaves him to die in the darkness while chained to a rock wall. Poe does something very similar in "The Murders in the Rue Morgue." He uses many words to discuss rational analysis and even talks about the difference between playing chess and playing checkers. Then he draws the reader into a tale which culminates in the most bizarre and horrible scenes imaginable--an orangutan killing two women with a straight razor and stuffing one of the bodies up a chimney. Poe deliberately moves from the coldly rational to the most harrowingly emotional for the sake of aesthetic contrast. Perhaps Montresor (i.e. Poe) does not discuss the "thousand insults" because that would involve strong feelings and these would be out of place in the logical, analytical introduction to the story.
Unredressed mean not remedied or corrected. When Montresor says a wrong is "unredressed" when retribution overtakes the redressor", he means that the injury Fortunato did to him will not be corrected if Montresor is caught for correcting the mistake. He wants to punish Fortunato, and punish him with "impunity". In other words, he does not want to get caught taking revenge.