How was the cultural upheaval of the 1960s related to the political and social changes of the decade? Is the "youth rebellion" best seen as a response to immediate events, or as a consequence of such longer term forces as the population bulge and economic prosperity? What were the long term results of the "counter culture" in all its varieties?

The "youth rebellion" was a reaction to both immediate events and long term forces. We'll deal with the latter first, primarily because the economic prosperity and demographic changes of the time have their origin before the events of the 1960's, when U.S. society underwent a huge transformation.
After World War II, there was an unprecedented increase in the general educational level of Americans. Men who had served in the armed forces were able to attend college on the G.I. bill, and millions of men who had come from working-class families suddenly became upwardly mobile, obtained high-paying professional jobs, and moved their own new families to the outlying areas of the cities to what were then new middle-class neighborhoods, and eventually to the suburbs. Their children, the baby boomers, grew up in circumstances far more comfortable and carefree than what the parents, the Greatest Generation men and women who had grown up during the Depression and experienced World War II, had ever known. Newer theories on child-rearing, such as those of Dr. Benjamin Spock, became popular. At the same time, however, the prosperity of the country was overshadowed by the Cold War and the ongoing threat of nuclear annihilation that was like a subtext to the entire culture. There was an uneasy paradox between prosperity and the possibility of instantaneous mass destruction, and the young people of the time were keenly aware of this.
It was natural that those of us growing during this time, the 1950's and early 60's, would begin to question long-standing ways of doing things and the values of our parents. Added to this was the Civil Rights movement. It was no longer possible to believe that the U.S. was a perfect country or to believe in slogans like "my country, right or wrong," for a generation which had the time and the luxury to be skeptical of the old ways, and who were witnessing, through television, the racial transformation that was just beginning in America. It's ironic, but not really surprising, that when children grow up in a level of comfort not previously known in the world, they are more likely to find things wrong with the establishment and to rebel against the attitudes of their parents and society.
Add to this the immediate events that convulsed the country, and which happened in rapid succession, beginning largely with the assassination of President Kennedy in November, 1963. During his short time in office, Kennedy was already becoming a symbol of change, a progressive force to whom young people looked up. The bottom seemed to have dropped out when he was killed. This was followed over the next two years by a massive escalation in the Vietnam War under Lyndon Johnson and a conviction, which many still have today, that had Kennedy lived, the war would not have been taken to such levels. The idea of young men being drafted to fight in a war half way around the globe when there was no direct threat (or any threat at all, as most people eventually could see) to the United States, seemed absurd. The slogans "better dead than red," and the assertion that the Communists were going to take over the world, came to seem ridiculous. Eisenhower, though much maligned for having been a do-nothing, boy-scout like President in the peaceful 50's, had warned about the power of the military-industrial complex. The Vietnam War went on year after year, with Americans dying, but no progress, no end in sight. The ability of television to bring the images of war into American homes, to show the corruption of the South Vietnamese regime we were propping up, and to show the domestic disorders caused by ongoing racial problems and by the unfairness of society to African Americans, made young people want to opt out of the whole "system." By 1968, when Richard Nixon was elected President on a platform that he had a "secret plan" to end the war, even most conservative Americans realized the war was a failure.
One last aspect of the transformation and upheaval taking place was the change in attitudes about sex, of course. The introduction of the birth-control pill in 1960, along with women entering the workforce in larger numbers and delaying marriage, brought about unprecedented sexual freedom and the destruction of traditional moral strictures about sex. This was long before the age of AIDS, and it was at a time when medications introduced over the previous twenty years, such as penicillin, had virtually wiped out, in comparison with the past, venereal diseases. Few of the past taboos about sexual behavior could survive in this wide-open climate.
By 1970, the U.S. was a much different place than it had been just seven or eight years earlier. Both long-term societal forces and changes going back to the end of World War II, as well as the immediate events of the 1960's, were responsible for this transformation.