When was the point at which the American Revolution became inevitable? Can you prove that it was always inevitable?
You are asking several ideas in your question. I will focus on the first question you asked about when the American Revolution became inevitable. It also is difficult to support the idea that independence was inevitable considering the time period of the revolution. There weren't too many models to follow that suggest independence was inevitable at that time.
There are two or three situations to examine to answer this question. For many colonists, the Boston Tea Party followed by the passage of the Intolerable Acts would be the point from which there was no return. When the colonists destroyed the tea, it was a big loss for the tea company. When the colonists refused to follow the Intolerable Acts, there was little room for negotiation. Both sides had dug in their heels.
Another possibility would be when the colonists began to form their own militias after the passage of the Intolerable Acts. This was a sign that they expected a war to occur. It shouldn’t be a surprise that unofficial fighting broke out at Lexington-Concord in April 1775. Once fighting occurred, it was pretty clear we were heading for independence and the war that would follow.
Some people might point to the Boston Massacre as a turning point. However, six years went by after the Boston Massacre occurred before independence was declared. Thus, there was still time to recover from this event.