Versions
Wish I could find a better source, but Wikipedia is what I have at hand at the moment; Spinola's wiki page is at odds with what you're posting. Says that he wrote the book in 1974. Not only that, he published the book after he was removed from his post. The MFA also started out as more of a protest against a 1973 military law.
I've also talked to a few Portuguese people about the Carnation revolution and it appears more along the lines that Spinola wasn't initially involved in it, but he became the representative since he was the only one that Ceatano would surrender power to. Of course this could be a view point formulated after seeing the April Captains, which paints that picture.
http://www.imdb.com/title/tt0120626/
There are many narratives about the 25Th April revolution. The one I gave can be said to be "the Army Version".
Remenber that career officers (as opposed to concripted or short term comission officers), being all graduates from the same military academy see themselves as being "family" and representing the Army. The diference btw a Colonel and a Captain, if they're both Academy graduates, is purely age, they share all the rest. Since the Movement was stricly a career officers thing it can be said to have been done by army "insiders". That was one of the reasons they would not fire on one another and there was close to no bloodshed even in the wild days of the summer of 75, when the comunists nearly managed to spilt the army.
After 75 a number of politicians tried to claim the "revolution" for their own colours, enhacing the role of the civilian oposition and minimizing the army role. Most active were the Comunist party hardliners, including many historians (the portuguese history colleges were bright red in the 70/80s). A lot of what you read, particulary on the web, comes from those people. The Army never claimed anything in return for handing over a democratic state to the people, going back quietly to it's units. Currently the radical left in Portugal, wich includes not one but the two comunist parties (one leninist and one trotskyst) with more elected members of parlamient in the world except for China, has made an habit of painting the April events as a sort of portuguese 1917...
Bear in mind that the Portuguese Comunist party was led in the 70/80 by probably the smartest and most charismatich man in portuguese politics in the XX century, Alvaro Cunhal.
Maria de Medeiros film is a romanticized versions of events.
Spinola was working solo even when he was still Field Commander in Guiné, working behind the scenes to try and get a peace deal (the guerrilas backed off at the last minute, some say on orders from Moscow, and killed the portuguese officers they had invited to a clandestine meeting)
Spinola's book, when it was finally published, was his way of issuing a final warnning shot to the Caetano regime. The Army had turned his way by then.
Caetano had to surrender because the national guardsmen holding the Carmo barracks would not stand to the Army. He was merely surprised to see that facing him was just Captain Salgueiro Maia, since he was expecting someone senior.
The military law you refer to was an atempt by the governement to reduce the control the career officers had over the army by promoting people who had not graduated from the academy faster and for longer terms. It helped alienate the career officers, but it was their conviction that colonial war, dictatorship and international isolation were ruinning the country that turned the army against the regime.
I might sound a bit to "hot" on the subject but for reasons you might deduce this is kind of a "family" issue for me...