The writer was Margaret Mitchell. It is OK to start with the germ of an idea for a story. But, make your life easier by building your characters first and letting their distinct personalities develop the plot with all the twists and turns for your story. If you have built your two main characters profile first on a separate sheet of paper you will know what makes them tick, what their ruling passions are, their fears, their goals, etc. This will make it easier for you to think up plot twists that actually play off their unique personalities and will help drive the storyline forward.
Character is plot, plot is character.
A. First, submit to the houses that take similar type books. Second, try trimming or throwing out your first three chapters. Start your story where the action starts, where the decision must be made, when the conflict is introduced. Many authors make the common mistake of using the first three chapters as 'get acquainted' time with their characters. This habit can cause a new author to be rejected because the average editor reads less than the first eight pages of a manuscript before making a decision to keep reading or reject the work.