Mercutio says Romeo is already dead because he believes Romeo's infatuation with Rosaline has left him lifeless and devoid of vitality and energy. Mercutio sees Romeo as being consumed by his love for Rosaline, which in turn has caused him to lose interest in life. He uses this metaphor to illustrate the extent of Romeo's emotional distress.
Not in the play.
Quite a lot really. Just about everything Mercutio says in Act I Scene IV is in response to Romeo's "heaviness".
Suppose that Romeo refused to take revenge on Tybalt for Mercutio's death. Maybe Mercutio's brother Valentine got there first. Then when Paris comes a-courting, Juliet could say, "Sorry, I'm already married." And her father would say, "Oh yes? Romeo, you say? Hmm, well he's a nice young man and this might put an end to this stupid feud once and for all."
Mercutio, who was being rude to the Nurse and she asked Romeo who that "saucy merchant" was.
Mercutio says that dreams are the product of an idle brain.
Romeo is deeper than Mercutio. Romeo is more romantic then Mercutio who has negative feelings towards love. Mercutio is a realist while Romeo isn't. Mercutio is into fighting while Romeo wants to talk things out.
When you say "mad" you mean "angry" not "crazy", right? Romeo experiences extreme anger in Act III Scene 1 when his friend Mercutio is killed in a swordfight with Tybalt. Romeo is especially angry because not only is his friend dead but Romeo feels that it is his fault. Mercutio blames him for causing his death by trying to break up the fight. Romeo's anger is even more at himself than at Tybalt, but he directs his self-anger to Tybalt, losing all control.
To say romeo wont get unlucky, and wont have bad dreams.
Mercutio says, "Thy wit is a very bitter sweeting; it is a most sharp sauce."
In Romeo and Juliet, Tybalt accuses Mercutio "Thou consortest with Romeo". To "consort" with someone is to hang out with them. That's at least one way of saying it.
In scene iv, Romeo's behavior shifts from being emotional and withdrawn to being lively and engaged when he teases Mercutio about his dreams and jokes with him. This change in demeanor prompts Mercutio to remark "Now art thou sociable, now art thou Romeo" because he recognizes that Romeo is returning to his usual sociable and witty self.
They sure did. In Act 2, Scene 4, the Nurse comes to meet with Romeo as Juliet promised the night before. She finds Romeo and his gang hanging around. Benvolio doesn't say much but Mercutio harasses the Nurse and constantly interrupts her attempts to converse with Romeo. He finally leaves, calling her "ancient lady"