"The Magicks of Megas-tu" established that some of our myths of demons and the like are based on the Megans.
My problem with "Adonais" is the timing. 5000 years ago is a couple of millennia too early for the heyday of Greek mythology. Unless the idea is that the myths developed by Greek civilization were a distorted remnant of a long-lost ancient history.
Of course not -- why would the aliens who were the basis of the Greek gods use the names of the separate Roman mythological figures that the Romans identified with them millennia later? Romulus and Remus don't even have counterparts in Greek myth.
The obvious intent was that those names were coined by humans. Vulcan was probably originally intended to be the Solar planet Vulcan that was once speculated to exist inside Mercury's orbit, as an explanation for its orbital anomalies. The idea of the planet was discredited early in the 20th century when Einstein's General Relativity was proven to explain the anomalies, but the idea of the Solar planet Vulcan persisted in science fiction for decades thereafter. Given that Roddenberry's original series format suggested that Spock was "probably half-Martian," he was likely assuming a Solar origin and intended Vulcan to be
that Vulcan, until his science advisors set him straight. But since Vulcan was portrayed as a hot desert planet, it was still implicit that the name had been coined by humans to reflect that quality of the planet.
Lora Johnson's
Worlds of the Federation proposed T'Khasi as Vulcan's indigenous name, while the Last Unicorn Games RPG proposed Ti'Valka'ain, which could easily be Anglicized to "Vulcan." But since
Enterprise, I tend to suspect that the Vulcans' name for their own planet is Minshara. (The novel
The Tears of Eridanus by Steve Mollmann & Michael Schuster listed all of those as alternate names for the planet in its various different languages, just as Earth is called Tierra, Erde, Chikyuu, Dijiu, etc. depending on the speaker's language.)
And the obvious intent in "Balance of Terror" was that humans named the twin planets Romulus and Remus after the twins from Roman mythology. The reasonable assumption was always that they had different names for their own planets. (In Diane Duane's novels, they were named ch'Rihan and ch'Havran, and the "Romulans" called themselves Rihannsu.) One of the stupidest things in
Star Trek: Enterprise was in "Minefield" where it was established that "Romulan" was their name for themselves. How culturally illiterate did the writers of that show have to be not to realize that the name came from Earth mythology? (On a par with the astronomical illiteracy of not knowing that Rigel is a real star.)
The basis of Von Daniken's beliefs was much uglier than that. The "ancient astronaut" theories he popularized were inspired by Nazism and the occult, and he believed ancient non-European cultures lacked the intelligence or drive to achieve great things on their own and thus must have needed pasty white aliens to show them how. The whole thing is just 19th-century eugenicism with space aliens tacked on, presented as pseudoscientific conjecture as camouflage for the implicit white supremacist messaging.
https://badarchaeology.wordpress.com/2014/01/12/is-pseudoarchaeology-racist/
http://www.jasoncolavito.com/blog/the-astonishing-racial-claims-of-erich-von-daniken
https://www.splcenter.org/hatewatch/2018/01/02/close-encounters-racist-kind