Thers no way the US would tolerate a communist Iran.
Beyond a certain point there is really nothing the US can do. This is just after Vietnam, intervention is politically as well as militarily unfeasible. It's a strategic country perched right on the Persian Gulf with a bevy of rich resources to match. The Soviets by the time of 1979 are getting into Afghanistan right next door. It was pretty much an unspoken rule of the Cold War that nobody puts boots on the ground in the Persian Gulf. Confrontation and quite possibly worldwide nuclear war are a strong possibility otherwise.
The revolution was anti-Shah, and as a consequence was pretty much anti-American. The Iranian people weren't idiots, they saw the writing on the wall and realized who was keeping their unpopular, despotic monarchy afloat. People who were pro-American had all of zero political credibility with the Iranian populace circa 1979. And it really only got worse after the Americans refused to return the Shah.
Iran had a strong and rather active leftist element to the revolution. The problem was twofold: they were disparate, with people running the gamut from democratic socialists to something that would be more amenable to the Muscovite brand of leftism. But with numbers and political organization alone, Iran should have been a leftist state after the revolution, but Khomeini and his supporters were a small, tightly-packed minority. They had a consensus and a singular purpose. And they also realized that if the leftists were going to fight the Shah, they would spend all their strength and leave themselves weakened for Islamist takeover in the aftermath.
The effects of this on the rise of Islamism as a viable political force would be enormous. Groups like the various chapters of the Muslim Brotherhood in the Middle East predate the Iranian Revolution by decades, but Iran was the first case where a group like this had successfully taken power. All the others were little more than theories: political entities of varying degrees of tolerance, but in the most prominent Arab countries were strongly suppressed or were simply co-opted by the state ideology, particularly in Syria and Egypt. Iran would still be regionally-isolated and seen as a threat: it's both revolutionary as well as ruled by the dreaded Persian tyrants. An Iran-Iraq War of similar but not altogether the same circumstances as OTL is not beyond the realm of possibility.