To whom is Montresor speaking (fifty years after the murder)?
It is my belief that Poe intended to have his readers assume that they were reading an English translation of an old letter which had somehow fallen into the author's hands. The original document could have been written either in Italian or in French. It might not necessarily have been sent to the person to whom it was addressed. Montresor might have written this "confession" one night while drunk and then decided not to send it the next morning. He had kept it among his papers, and it was found after his death.
Since the story is written in English, it would detract from the verisimilitude if the reader had to assume that Montresor was actually speaking to someone but that he was really speaking French or Italian. Furthermore, the document sounds too precise for a narrative being spoken to another person or group, especially since the incidents are being remembered fifty years after they occurred. So in answer to the question "To whom is Montresor speaking?" I would reply that he is not speaking to anyone but is writing a letter. Otherwise we would also have to suppose that we are somehow eavesdropping on a man who is living in a foreign country, talking in a foreign language, and is probably already dead.
I do not believe Montresor is trying to clear his conscience after fifty years by confessing to a murder. I do not believe that he even feels guilty about what he did to Fortunato. In fact, the successful execution of his plan for revenge seems to have relieved him of the hatred and anger he felt. He hated Fortunato intensely fifty years ago, but he does not, and cannot, hate him now. At the beginning of the narrative Montresor talks about the requirements for a successful revenge. He does not say that a truly successful revenge would accomplish its main purpose, which is to get rid of the painful emotions that lead a person to consider committing a murder. But this cleansing of emotions, this satisfaction, this closure, is the most important part of revenge (providing the avenger doesn't get caught). When Montresor ends his narrative with the Latin phrase meaning "Rest in peace," he is being totally sincere, not ironic. He has nothing but good feelings for Fortunato since he has evened the score with him.
Poe leaves it up to the reader to make that dicision. Could he be speaking to a group of friends, being a braggart, or maybe he is on his deathbed confessing to a priest his sins. In a lot of literature it has to do with what you bring to the work, your experiences, your beliefs, your values... in the words of a professer I had, literature is a personal experience and what you bring to it is different for everyone. I believe this to be the case here, it is left up to our own personal interpretation. Personally considering that at minimum our offender was in his 20's, 50 years later he would be in his 70's. Therefore it is my belief that as Montresor lay dieing, he confesses himself to obsolve himeself of his sins, taking also into consideration the setting would allow for a prominent base of Catholisim.