Darcy also suggests, and elsewhere it may be taken for granted, that Americans called for the coup, or at least were ultimately responsible for it. Mourão Filho kickstarted the movement without even coordinating with conspiring officers in other states and the American embassy was trying to keep track of a movement it did not control, but the wider question of whether Goulart would fall without American hostility is important. I'm of the opinion that he still would.
For a few years there had been a degree of American subversive activity (I've even found the claim that corporal Anselmo of the sailors' revolt was part of subversion) and support for right-wing organizations. Through training courses America had also fostered anticommunism in the officer corps.
But none of this was fundamental. Political divisions were so deep because of society's internal development -as the Marxists say, of the economic situation and the state of class struggle. Internal societal conflict was what caused the coups in 1889, 1930 and so on, not external interference. Maybe Cold War polarization helped but something on the line of "reactionary landowners act to prevent land reform" is internal.
On execution, too, internal forces managed to overthrow the old regime by themselves. Knowledge of American opposition did influence Goulart's decision to fly to Brasília, and ultimately to exile, but it was still possible for the rebels to put enough pressure on him to do that without the American factor.