Were pre-revolutionary war, grievances to the king valid?

Answer 1

Yes,
There was a major taxation problem. In the British Constitution at that time, which is actually a compilation of documents from over several centuries, there was a clause that stated that all taxation requires representation in one house of parliament. Representation was poor in Britain itself since seats went to traditional families or were bought and sold. If representation was bad in England, it was non existant in the colonies. The new world had some benefits that other British colonies didn't have. They had colonial legislatures and governors that they elected themselves. That wasn't present, say, in India. However, King George, who later went insane, continued to raise the taxes in the American colonies because he had drained the treasury for the French and Indian war. These taxes were made without the representation that their Constitution allowed for and they were ridiculously high on the necessary items, like all sugars and any paper products (cards and stamps as well as any paper.)
In short, the claims of the founding fathers were valid. They had a political standing as well as the general complaints of the people. It would also be true to say that the revolution made things harder for a time. It wasn't until the singing of the Constitution in 1789 when things got much better for them. The articles of Confederation made trade difficult, too.

Answer 2

yes and no
britain assumed a great debt saving us from french rule during the french and indian war and thought the colonies should repay it. however the "no taxation w/o representation" is valid as well. the colonies only had a form of "virtual representation"

Answer 3

yes. If he had treated the American colonies like he treated the Canucks, the war would have been adverted and we would have stayed under his control.