The purpose of King's letter is to cause the Church to fight racial injustice.
As you already know, King was a proponent of equal human rights, for both black, and white people. He wanted the Church to be aware of inequalities in the world and fight racism so as to erase those inequalities.
I'm going to say "It set up the events necessary for the surprising conclusion." :)
Lizabeth understands the destroying of Mrs. Lottie' marigolds as her final act of childhood, the final act of innocence.
Lizabeth feelings that led her to destroy the marigolds were "the great need for my mother who was never there, the hopelessness of our poverty and degradation, the bewilderment of being neither child nor woman and yet both at once, the fear unleashed by my father’s tears".
The story is situated during the Great Depression. Her mother is never home because she has to work, her father cries because he can't provide for his family. You add the hopelessness of their poverty and the fact that she is going through defining times between being a woman and a child she doesn't understand at the moment, she must have felt confused and lonely, which leads to the destruction of the marigolds as an impulse she can't control.
Before she has stated that she hated those marigolds because they have the nerve to be beautiful in the midst of ugliness, they didn't match with the house, the times, and what she was feeling inside.
It is a myth because no one knows anything about it.
