I think that any etymology of "Yo!" that goes back only a few hundred years is woefully incomplete and quite absurd.
"Yo!" is used in more-or-less formal situations in East Asia (China, Japan), India (Dravidian languages), Africa (West and Central Africa), the United States, and Europe. That usage range puts it well beyond the purview of Indo-European, and suggests that its origins could lie entirely outside any formal etymology - but if it does have an origin, it obviously ain't English (as your source up there says, suggesting it may have come from Africa, or the Mediterranean, or both).
Arguing that this simple sound is derived from "an exclamation" back in AD 1400 is saying nothing more than "Back then, in AD 1400, nobody knew where it came from, either." Compare, for instance, the exclamation "Zounds!", which has a certain date of origin, and a certain meaning from which it is derived: "Yo!" has none of that.
Basically, "Yo!" is a simple sound that gets used a lot, around the world; so long as it's not a formal word in one's local language, it will tend to get used for more-or-less formalized exclamatory purposes. This makes sense because it's A) easy to say, B) the sounds occur in pretty much any language on Earth, and C) the sounds carry a quite a way's distance, and are easily distinguished from other sounds and words.
In the US, it was re-purposed as a greeting and response by Af-American culture some time in the late 60's, or so, and that's the answer you really want, here. It may have been absorbed into Af-American culture through Basic Training in the US military, during Vietnam (or WWII, as suggested by the other poster, above), or it may be a holdover from something more ancient, perhaps an African dialect; it's to answer questions like this that the idea of "ebonics" was once promoted. I have no idea if that discipline -- if it can be called that -- is still around or not, but that might be a good place to start if it is. In any event, it appears that currently linguists just can't really give your question any definitive answer.
You have it pretty much spot on, let me fill in your blanks in reverse order, so as to be in chronological order, and to end with the bit you describe as "awesome".
You were correct not to be surprised; it's Italian and like many English words of Italian origin, relates to music being originally an opera term.
An interesting early use in this regard is from George Grove;s A dictionary of music and musicians:
Scenario, an Italian term, meaning a sketch of the scenes and main points of an opera libretto, drawn up and settled preliminary to filling in the detail.
Interesting, because it shows us that in the 1880s it was being used in English, but still noted as "an Italian term".
Now, you ask:
did it experience another shift of meaning in 1910?
Yes, the cinema! As cinema moved from its early infancy into being a narrative art, it took the term scenario and applied it to what is now called a shooting script.
That sense has now died out because...
So, did "scenario" experience a shift in meaning in 1960?
Yes. As you noted:
it became used to describe nuclear outcomes during the Cold War
More specifically, in the hey-day of the think tank in the aftermath of World War 2, Herman Kahn was working for the RAND Corporation on applying game theory to strategic planning for the potential of conflict between NATO and Warsaw Pact countries with their potential to involve nuclear warfare.
He was heavily involved in producing an approach to thinking about plausible outcomes rather than just those deemed likely, and planning accordingly. His approach involved writing scripts for (so-far) fictional events, as if written by people in the future. Since this was essentially taking the fiction-writer's approach to practical ends, he took the term scenario from the dramatic arts, and so he is considered one of the inventors of scenario planning and almost certainly where it takes its name.
While best known now as one of the inspirations for "Dr. Strangelove" his book Thinking the Unthinkable* had some popular success and was much read by journalists in particular. From this scenario in his specialised use, in which it was essentially a piece of military strategic jargon, moved into the popular culture, and from then became increasingly loose in meaning.
Kahn went on to write crazy predictions of how by the year 2000 we'd have new forms of birth-control, sex-change operations, widespread peaceful use of nuclear power, real-time banking and phones that fitted in our pockets. (Clearly a mad-man).
He'd done his bit for scenario, turning it to a jargon use that mutated into the form we know. Thinking the Unthinkable was published in 1962, which you'll note comes in just before the rise in the graph in google ngram.
*Which amusingly enough cites Red Alert, the novel that Dr. Strangelove was a loose adaptation of. Reading it now it's hard not to picture Dr. Strangelove using Dr. Strangelove references.
Best Answer
The Facts on File Dictionary of Proverbs has the following (emphasis mine):
Here is the relevant quotation from Haliburton's book, completely titled The Attaché; or, Sam Slick in England, and courtesy of Project Gutenberg:
Haliburton was a novelist, and Sam Slick seems to have been his primary literary foil, much like Hercule Poirot and Tom Ripley became for Agatha Christie and Patricia Highsmith, respectively. From Wikipedia (emphasis mine):
The opening paragraph from a biography of Haliburton, titled Inventing Sam Slick: a biography of Thomas Chandler Haliburton, gives one a clue to Slick's renown:
So our best guess should be that the sentiment talk is cheap spread in its current form as a result of being associated with a long past literary phenomenon in Sam Slick, whose turn of phrase — perhaps even including talk is cheap — seemed to have delighted those over the pond.