The declaration was a laundry list of affronts to the colonies by King George III, wherein Jefferson et.al. argued that since the monarchy did not treat the colonists like other Britons, the colonists had the right to form their own sovereign nation. It was also intended to justify this action to other monarchies to prevent them from helping Britain retain the colonies under the doctrine of Intervention. So it helped to justify the war domestically and internationally.
Agriculture was the main factor
Because they got tired of oppressing south.
South was also tired of military rule. As the South got more and more upset about the military rule, North came under more pressure to end it.
The only reason for military rule was protecting the black rights. Most of the North didn't care abot black rights.
This was the reason why reconstruction ended.
B. A vote in the Continental Congress
Hope this helps