121: “The I of the book Cannot die in the book“: refers to the immortality of the characters of a novel, focusing on himself as this novel is an autobiography. Any character in a novel is immortal because a reader is able to reread the novel whenever he/she wish to. By adding this quote, Nabokov wants the reader not just read one novel once but reread it.
121: Planchette-Fiction: a planchette is a board that was used during Victorian era to facilitate automatic writing. Many people believed that the device was used to communicate with the dead/spirits. This was used to fool the audience. Nabokov may be referencing this to suggest that he is fooling the audience as well.
121: “I found myself standing on a staircase of onyx before a great gate of gold in a crowd of other bald-headed angels“: This can be reference towards Herbert Gold, an interviewer. Nabokov despised the man for telling anecdotes about Nabokov calling him a liar, hypocrite, untalented etc. Nabokov has been infamous known for sparring with reviewers if they wrote something that Nabokov disagreed with.
121: me, an old gentleman, a distinguished author: Can be a reference towards Vladimir Nabokov himself
121: paresis: is a condition typified by a weakness of voluntary movement.