Is “Was Zhang Song's exposure due to him drunkenly dropping a secret letter that was picked up by Zhang Su's attendant?” true?

Not so. The official history 'Records of the Three Kingdoms, Book of Shu, Biography of the Former Lord' records: 'Song's elder brother, Guanghan Administrator Su, fearing disaster would implicate himself, informed Zhang about the plot. Thereupon Zhang arrested and executed Song.' The official history only records that Zhang Su, out of fear of being implicated, reported his younger brother. Chapter 62 of 'Romance of the Three Kingdoms' fabricates the plot point that Zhang Song 'accidentally dropped this letter on the ground, and it was picked up by Su's attendant,' rewriting the simple act of self-preservation into an exposure due to carelessly dropping the letter.