The tl;dr of American intervention is that
-American leadership was to the highest levels favorable to Goulart's overthrow
-A prolonged rebellion was likely to face logistical difficulties
-Americans had preparations for sending a task force and oil tankers to provide logistical support
-America was worried about legitimacy and wanted a rebellion to have overt civilian support and a legalistic basis
-It was also reluctant to commit to a failed rebellion
-The ships did set sail
-Goulart's camp had a notion that something was going on, but very vague and second-hand through connections to the Minas Gerais state government.
-Ambassador Lincoln Gordon and military attaché Vernon Walters kept track of what was going on
-As the rebellion seized power on its own the ships turned around and the topic was forgotten until the files were dug up

Gaspari covers this in detail. He states clearly that the ships were never in Brazilian waters. The claim that they were 12-50 nmi off Vitória does not hold to scrutiny. It is in contradiction with the quoted American document, which besides speaking of immediate dispatch and that the ships were not in the area, gives the destination as Santos (i.e. to supply a rebellion in São Paulo, nothing for Vitória) and the dates of arrival as after the 10th of April. If only the timetables are taken as valid and the other points disregarded, it still doesn't click.
https://www.geografos.com.br/viagem-maritima-entre-portos-brasil/distancia-entre-porto-santos-e-porto-vitoria.php
It takes 1 day and a half for a cruise ship to set sail from Vitória and arrive in Santos. For the slowest option considered, a sail ship, that is 4 days. The timetables, if assumed to be for a ship off Vitória, give a travel time of 10+ days, which would mean a speed lower than that of sail ships!

Gaspari is also explicit that there was no plan to send troops, and that doesn't show in the documents. Yet Darcy Ribeiro is no random source: as Goulart's most powerful advisor he was directly involved in the power struggle within the two days of rebellion. What I do assume is that, despite stating his claims are based on American documents, his statements on how precisely American intervention took place are second-hand. He doesn't go into detail about this while Gaspari even starts paragraphs with sentences such as "At 17:38 Lyndon Johnson's telephone had rang again on his ranch in Texas". Gaspari looked up sources by himself and one of his sources was Darcy Ribeiro himself; in one case an incident was written vaguely in Darcy's memoirs but expanded on Gaspari's writing by what the former had confided to the latter. Furthermore despite being a central source Darcy may be biased.