A.) The war was justified from the colonial point of view.
B + C.) The primary cause of the war was the Currency Act of 1764, when Great Britain prohibited the Colonists from using their own currency, Colonial Script.
Colonial Script was debt free and helped the colonies prosper while the centralized Bank of England was exploiting British Economy.
The financial powers beyond the throne pressured King George to issue the act, which almost instantly ruined the economy for colonies. The colonists would've been happy to pay a little tax on tea and stamps had they been allowed to use their own money; but England demanded that the Colonists borrow their money from England. As British money was already tainted with a failing economy, it is understandable why the Colonists found this oppressing.
Relations soured even more after the Boston Massacre.
After 10 years of a failing economy, secret societies within the colonies such a Freemasonry, influenced public opinion into seeing independence as a reasonable alternative. Indeed, it was Freemasonry that instigated the Boston Tea party. War was impending at this point and it was when the Colonists declared their independence that was last straw.
D.) There's always a possibility for diplomacy, it would require give and take on both ends; however the British bankers weren't allowing to give the colonies an inch.