(1) To what extent did this US involvement help propel the Dalai Lama as an internationally renowned spiritual leader?

As far as I know and can tell, basically not at all. The Dalai Lama was essentially an unknown figure outside of Asia until 1989 when he won the Nobel Peace Prize. 1989 was, also, the year of the Tiananmen Square Massacre and it's generally believed that the award of Prize to a major opposition to Beijing's rule and authority was a repudiation of the People's Republic's actions at Tiananmen Square specifically, and towards major Communist powers generally (as the USSR was also collapsing, there was some belief, though how thoroughly and extensive I don't know, that China might have been next. Certainly the Chinese government feared that anti-Communist factions within them might be emboldened).

Anyway, it was the 1989 Nobel Peace Prize that promoted the Dalai Lama into an international figure of fame unlike any other even before, including the 1950 annexation of Tibet to the People's Republic, or his 1959 flight to India following the uprising in Lhasa. It even, somewhat ironically, promoted him to a position of global prominence among Tibetans who previously held their own local cultures (Amdowa, Khampa, etc.) above a united Tibetan identity, and the 1989 Nobel Peace Prize, promoting both the Dalai Lama and the Tibetan Cause instilled a new sense of national and cultural pride among Tibetans, and a new generation of Tibetans bore the Dalai Lama's first name, "Tenzin," directly from this event.

(2) The program and its support also for Tibetan guerilla forces seems to have been cut after the 1972 US-China approximations - but were there other long-lasting consequences of this program?

Not really. While the CIA maintained some form of contact with the Dalai Lama, basically throughout the 1950-1972 period, he was not quite the asset that the PRC would like to make him out to be. The CIA approached the Dalai Lama in 1950 as the People's Liberation Army was marching on Lhasa. They told him they would provide weapons, money, and political support to an independent Tibet. There were essentially two preconditions for that support: 1. He had to go into exile, either in India, Ceylon (modern Sri Lanka), or Thailand. The closer to Tibet, the better, but these were apparently the three options presented to him. The first being Tibet's biggest supporter in the international community, the other two being deeply Buddhist countries and allies in the fight against Communism. And 2. He needed to maintain the principle of Tibet as an independent state and Tibetan sovreignty. The Dalai Lama has been open about this meeting (and it's worth noting that he was fifteen at the time (!!!)) and agreeing to these two conditions essentially meant plunging Tibet into a protracted guerilla war. As I detail in this writeup, the United States would have been more than happy to have Tibet open up a second front against Beijing during the Korean War, but the young Dalai Lama was not eager to plunge his country into a war that few people believed they would even win. That, and probably drawing inspiration from his predecessors, he went back to Lhasa to deal with the Chinese commanders who had just militarily conquered Tibet. It was a troubled relationship, until 1959 when it broke down completely and he left for India, repudiated the Seventeen Point Agreement that his government signed under duress, and maintained somewhat a principle of Tibetan independence and sovreignty, while later amending his political philosophy to the "Middle Way" approach, envisioning a Tibet that was united, with its own justice system, language, religious traditions, but held the Beijing government as its suzerain. China's opinion and reaction to this concept is beyond our scope or interest here.

The CIA, meanwhile, had to mostly move on without the Dalai Lama. They found some Khampa leaders, flew them to Guam, trained them in modern weaponry and communications, air-dropped them into Kham, and the insurgency in Kham became somewhat self-sustaining (as far as I can tell). I certainly haven't reviewed the CIA documents themselves, and I haven't read The CIA's Secret War in Tibet by Kenneth Conboy and James Morrison (though it's been on my shelf for forever), though I do know that Tibetan guerilla forces captured caches of Chinese weaponry which were regarded as huge boons to the guerilla fighters. Somewhat obviously to me, this indicates that while the CIA was certainly supportive of the Tibetan cause, their money and guns had their limits. Especially since Kham (western Sichuan Province, eastern Tibetan Autonomous Region) was pretty difficult to get to using 1950s and '60s technology, with unfriendly or outright hostile territory the only way in or out of the country.

(3) When did this US involvement in Tibet become public knowledge and did it create a scandal when it was revealed/declassified?

I don't know exactly when it became public knowledge, but there seems to have been essentially no scandal (compared to the CIA's involvement in, say, Cuba, their involvement in Tibet can be described as milquetoast at best). The Dalai Lama was open about meeting with the CIA in his memoir My Land and My People is pretty open about this initial meeting (published in 1962), and the issues revolving around the CIA's (and more generally the USA's) involvement or lack thereof in Tibet is pretty well documenty with the diplomatic and international issues in helping the Tibetan situation. It certainly didn't create a scandal as the Chinese government would have been the only one to really care, and if they didn't know (it seems like they did) it was certainly suspected and assumed. A condition Mao gave to Nixon for the normalization of Sino-American relations would have been the ceasing of support to the Khampa guerillas.

Sources:

Tenzin Gyatso, My Land and My People

ibid., Freedom in Exile

Sam van Schaik, Tibet: A History

Tsering Shakya, The Dragon in the Land of Snows: A History of Modern Tibet Since 1947

Warren W. Smith, Tibetan Nation

ibid., China's Tibet? Autonomy or Assimilation