In "The Gift of the Magi," is the plot twist most important element of conclusion?
A good question. Unfortunately, answering it requires hedging. For the plot, the plot twist is definitely the most important element of the story. Without it, there would be no real point to the story, and O'Henry's masterpiece would have been forgotten long ago. However, if you want to judge the story by the themes and meaning, then the most important element is Jim's response to the twist (he recognizes it as love, not tragedy), and the final summation: " But in a last word to the wise of these days let it be said that of all who give gifts these two were the wisest. O all who give and receive gifts, such as they are wisest. Everywhere they are wisest. They are the magi."
Do you mean the unusual twist of events? In that in giving, they "undid" the usefulness of the other one's present? If so, is an great example of dramatic irony, a form of irony in which one is led to think one thing will happen when finally the opposite thing occurs instead.
Note that in stripping away the usefulness of Della's and Jim's respective gifts, O. Henry emphasises the significance of their gift-giving (an expression of devotion, sacrifice and love) instead.