I had to edit your question because you had several questions that need to be addressed individually. Enotes editors are only allowed to answer one.
One relevant quote that shows Gatsby's belief that Daisy will leave Tom occurs in Chapter 7. Gatsby confronts Tom:
"I told you what's been going on . . . . Going on for five years--and you didn't know . . .But both of us loved each other all that time, old sport, and you didn't know."
Then Gatsby states to Daisy, "You never loved him."
The whole point of going to the Plaza Hotel is to inform Tom that Daisy is leaving him for Gatsby.
However, this is clearly an illusion. Daisy has no true intention of leaving Tom. Early in the novel, the reader becomes aware of the strong bond between Daisy and Tom when Tom's mistress calls him at the house, and Tom and Daisy have words. Even after this episode, Nick leaves the Buchanan mansion believing that Daisy "had asserted her membership in a rather distinguised secret society to which she and Tom belonged" (Chapter 1). This closeness foreshadows Daisy's own relationship with Gatsby. Even though Daisy has an affair with Gatsby, she, like Tom with Myrtle, will not break off their marriage. So in Chapter 7, when Gatsby is just about to inform Tom that Daisy is going to leave him, Daisy tries to interrupt Gatsby and change the subject. Even though clearly Gatsby and Daisy talked about running off together, when the moment comes, Daisy balks.