What do Lavinia and Edward choose?
Instead of choosing to go their separate ways, Lavinia and Edward choose to remain in their marriage and try and figure out how to live together. Edward makes the first step towards this when he asks the Unidentified Guest to bring Lavinia back to him. Then, they work with Reilly to get to the bottom of what has been preventing their marriage from functioning properly. They learn that they each fulfill the other's deepest fears. Lavinia fears she is unlovable, while Edward fears he is incapable of loving. By understanding and acknowledging this unspoken arrangement, they are able to move on and embark on a more equal, self-aware, and compassionate marriage. While it is not necessarily especially loving, their marriage becomes amicable.
What does the cocktail party symbolize?
The titular cocktail party is both a setting for the action of the play as well as a symbol for the ways that human beings must reconcile their private lives with a public audience. Edward does not tell his party guests that Lavinia has left him, but later explains it to the Unidentified Guest, worrying about what his friends would think if he told them. Then at the end, Lavinia and Edward anticipate the start of the cocktail party they are going to throw, while looking forward to the opportunity to get away to their country house and spend some time alone. The public world of the cocktail party is a necessary part of polite society, but it is not necessarily something that anyone enjoys.
What is the irony of Edward going back to Lavinia?
Edward has been having an affair with Celia, and even tells Celia that he believes she is the only woman he has ever loved. Thus, it is ironic that he chooses to discontinue their affair and go back to his wife. While one would expect someone to want to marry the "only" person they have ever loved, Edward would rather maintain the status quo and make a normal and uncontroversial life with Lavinia.
How is Celia's fate at the end of the play a racist and colonialist fantasy?
At the end of the play, Alex reveals that Celia has been killed by the indigenous islanders where she was working as a missionary and nurse. The characters mourn her grisly death, but suggest that she died nobly, trying to help people who needed her. The play imagines a white woman dying at the hands of the "savages" that she is trying to help, and suggests that it is somehow a noble death. This depiction of colonial intervention assumes the inferiority of the colonized people, and suggests that colonial missionary intervention is only ever a gift to the colonized people rather than an excuse for an empire to take from the colonized nation.
Is the ending a happy one?
It is difficult to say whether the end of The Cocktail Party is a happy one. In a superficial way, the central characters, save for Celia, end up pretty happy, content in the lives they have chosen. According to Reilly, none of the characters who remain within society have chosen the nobler and more difficult experience of self-actualization that Celia did, but they have found a kind of equilibrium that resembles something like happiness.