We know from the book that Mek Jokkor is not Vulcan (which is my only
thoughts on why Spock is unaffected), but it is possible that Zarv and
Let me offer a few quotes from the book. Here's one showing Kirk's first impression of Zarv.
"Kirk?" demanded a short, piglike man. "When can we start for Ammdon?
Time is of the essence in this urgent matter. We must not delay. Not
"Ambassador Zarv," Kirk said. The Tellarite seemed an unlikely choice
for negotiation of any type. He was brusque, rude and going out of his
way to be obnoxious. "Welcome aboard the starship Enterprise."
So we see that Ambassador Zarv was neither human nor Vulcan; he was a Tellarite. I would hazard a guess that his species has brains which are simply "wired up differently," so that sonic manipulations which tend to work beautifully on humans (and likewise on typical members of several other humanoid-looking species) don't do a heck of a lot to affect Zarv's emotional condition. If he ever did feel some slight impulse to trust Lorelei and agree with everything she said, it wasn't anywhere near an "overwhelming" impulse, so he didn't have to worry about it.
That's in addition to his being a strong-minded fellow and an experienced diplomat -- i.e. he'd heard lots of charming voices, from lots of different species, usually saying lots of interesting things . . . without his being mesmerized by any of them.
And here's Kirk's first impression of Mek Jokkor:
“And the other member of our team is Mek Jokkor. Mek Jokkor’s an
expert on agricultural products, especially those cultivated in the
Orion Arm.” Kirk shook hands with Mek Jokkor, felt a slight stickiness
when he pulled his hand away. “Mek Jokkor is not animal, such as we
are, Captain. No DNA. He is more closely related to the plants of our
world than he is to us.”
“You don’t speak?” Kirk asked, staring openly at the being. A tiny
shake of a human-appearing head was all the answer he got.
In other words, Mek was more of "the vegetable kingdom" than "the animal kingdom." I presume that whatever his species uses as their rough equivalent of a "central nervous system" works very differently from those of any of the flesh-and-blood species who are particularly susceptible to the things Lorelei can do with her voice.
Incidentally, while I was rereading the first chapter of the novel, I noticed that Kirk's first impression of Donald Lorritson was that he appeared to be from Earth. (When I see a name like "Donald Lorritson," I would only be surprised if he didn't have terrestrial ancestry!) In Lorritson's case, I suspect that you were right about "steadfast in his ways" as the implied explanation for why Lorelei could not quickly persuade him that Zarv's mission was a fool's errand.
(Or, to phrase it a little differently, the writer obviously didn't want any member of Zarv's team, not even the human one, to rapidly defect to the other school of thought in response to Lorelei's speech -- so he simply made the tacit assumption that Lorritson must be exceptionally resistant to the lady's hypnotic voice. Having done so, he quickly kept the plot moving forward from there, instead of bothering to dwell on that little point and try to justify it with a detailed explanation for the reader.)