I want to know what the story says about the theme of disguise with regard to the relationship between truth and disguise
The fact that Montresor wears no disguise and at the same tiime conceals his true intentions tells us that he is always in a disguise - looks like a sane man but in fact, is not.
Actually, nowhere in the story is there anything explicitly stated about disguise, except of course reference to Fortunato's costume. Perhaps the narrator wears one, too. Some old engravings depict the story that way. We could presume this since the story takes place during the Carnival season in Italy.
On the other hand, the narrator has much to say about disguising his intentions. The reader knows from the very start that Montresor is going to tell a tale of revenge. He talks about how he "baited" Fortunato to come inside his family vaults, which served the double purpose of cellar and tomb. He is proud of his skill at deceit, and there is no hint of remorse when he reveals how he went about his crime.
About the character roles - there an analogy between Montresor and Fortunato; Montresor is the king's "jester," so to speak, and Fortunato is the king's "fool." Montresor "plays" with his victim before killing him and Fortunato, in his naïve stupor, is the idiot who doesn't pick up on what is going on....
You've got a good start with the comment you have made under your initial question. Montresor uses the community celebration for his affront on Fortunato since Montresor knows that Fortunato will be drinking and therefore easier to convince to come see the rare cask of wine he has in his cellar. The celebration and the noise it creates is one disguise...no one will be able to hear Fortunato's screams, and if the screams are heard, no one will think anything of it since many people are screaming. In addition, the internal celebration Montresor is having thinking about getting rid of his foe is another disguise. He puts on the face of a friend, of someone who shares an interest in Fortunato's love of fine wine, but in reality his intentions are much darker. The wine cellar is also wearing a disguise...or at least serves a dual purpose. It is meant to hold and cool wine, but it is also a final resting place...the bones along the walls suggest that Fortunato isn't the only one down there. Then there is the issue of Fortunato's name. His disguise is one of fortune...good luck...yet his demise suggests something quite the opposite.
What does the story say about the theme? Well, for one, it suggests that one should be more aware. Montresor gives Fortunato multiple hints and chances to leave his company. The story also suggests that one should be careful whom he/she chooses as friends.
In "The Cask of Amontillado" Poe may be telling the reader that the greatest deception is what appears to be truth, especially if the perpetrator of the action is deluded himself and the victim is somewhat foolish. And, as Hawthorne remarked in "The Scarlet Letter," no man can wear another face for so long that he does not become confused about which one is truly his. Montesor believes that he has endured a "thousand injuries," yet there are no specific examples provided the reader. Thus, the entire narrative may be predicated upon the self-delusion of the narrator, in whose mind the plan is justified. Willing to accept this plan is Fortunato, dressed in the motley of a court fool. He, too, seems delusional as he fancies himself the greatest of wine connosieurs and does not realize what danger lies ahead.
Thus the truth is skewered throughout the narrative with ironic twists on statements such as Montesor's "you should use proper caution" and to the very end with Montesor's feigned concerns for Fortunato's health as he leads the motley Fortunato into the trap. Even at the end of the story, there is a double-entendre with the truth--"For the love of God, Montesor!" "Yes...for the love of God!" and "truth" continues its duplicity and deception.