I think Spock is his first name (a deleted scene of the first movie also made it so), it's his last name/family name that is difficult to pronounce.
You're making a Western assumption there, that the family name must be the last name. There are many cultures on Earth that put the family name first, like China and Japan. And there are Trek aliens that do the same, like Bajorans.
In fact, I don't know where the common idea came from that Spock's
first name was unpronounceable. I used to assume that myself for some reason, but the actual exchange in "This Side of Paradise" is:
Leila: "You never told me if you had another name, Mister Spock."
Spock: "You couldn't pronounce it."
Then, in "Journey to Babel," after Kirk addresses Amanda as "Mrs. Sarek," she asks him to call her Amanda and says, "I'm afraid you couldn't pronounce the Vulcan name." From context, she must be referring to the family name, the name she would have adopted as the wife of Sarek.
Taken together, then (and given that both scripts were by D.C. Fontana), we can surmise that it's Spock's family name that's unpronounceable to humans, and it's unspecified whether it comes first or last.
I mean, sure looks like a nicer 'maternity ward' than the STV's fucking dirty cave. Assuming the STV scene was meant to be a projection of the 'real' event, and not Spocks daddy-issues tainted imagining. I'd lean towards the latter. Even Sarek wouldn't be stupid enough to react that way whilst standing right there in front of a no-doubt tired and touchy Amanda.
The deleted scene of Spock's birth would've taken place before Nero's incursion, and thus would've been set in the Prime timeline, unaltered from the original history. So it was meant to represent the genuine event -- although, since it was deleted, it isn't official canon.
Who was the writer that ended up picking the EU name?
If you mean S'chn T'gai, that was first coined in
Ishmael by Barbara Hambly. That novel isn't really counted as part of the modern book continuity (in fact, it's kind of offbeat, since it's an unauthorized crossover with the Western
Here Come the Brides and has cameos by a bunch of other Western and sci-fi characters), but the family name (and the name of the clan, Hgrtcha) was referenced by Steve Mollmann and Michael Schuster in their
Myriad Universes tale
The Tears of Eridanus. That was set in an alternate timeline, but the MyrU tales are generally assumed to be "real" events in the same multiverse as the main novel continuity, so I referenced the family name myself in
Rise of the Federation: Uncertain Logic.