The plot is basically an outline of what will happen in the story so 'Plot - Establishes an order of events'.
The setting is where the story takes place and it sets up the mood in a way e.g. if the setting is in a haunted mansion then the atmosphere would be tense and edgy so 'Setting - Contributes to the mood'.
Conflict is the problems that causes issues. These issues usually affect the characters and this can lead to fighting between characters so 'Conflict - Builds the tensions between or within characters'.
No character = no overcoming problems = no story, therefore 'Character - Acts to overcome obstacles'.
The theme is normally some sort of message or idea that keeps coming during the story e.g. if the story takes place in a war-torn country with the character overcoming the problems that come with living in that country then the theme would center around war and the effects of war. For that reason 'Theme - Conveys a message or truth about life'
There were several reasons why, the American Union would be more powerful. And also because then black free men were disrespected and Douglass wanted them to seem worthy of American citizenship. If black men were allowed to fight Douglass believed that since they risked their lives for their country they would be worthy of citizenship