Thanks for your question. To be honest I had to research into this as none of the known adepts or initiates has to my knowledge ever mentioned the Pleiades as origin of beings such as those called Pleiadians.
The short answer to your question is that the stories of extraterrestial beings from the Pleiades stars has been ‘made up’, but as often stories are a mixture containing elements of truth and other origins, and indeed it is said that half truths are worse or more dangerous than lies.
You rightly intuit the Pleiadians are “some kind of fabrication, a mash up of truth”. Though you write Researchers of the Truth or anthroposophists should not deal with it, I disagree and would say there is a responsibility to use one’s knowledge and discernment to qualify such misguided stories. It doesn't mean we should spend more time on the topic though as all distractions come at the cost of not focusing on where we ought to put our energy.
But there's always the learning opportunity, and it's always interesting to understand what is going on, why these stories appear and on what they are based.
On the Pleiades
First, as you know, the Pleiades star cluster in Taurus has been part of ancient myths of cultures around the word. In fact the name Pleiades (or seven sisters) stems from Greek mythology. However we find the seven sisters or boys in most ancient traditions, including native american, japanese and chinese.
The Pleiades are thus part of humanity’s storytelling and the Mysteries across cultures and ages. For example Rudolf Steiner states:
1910-12-18-GA124
Take, for instance, the seven holy Rishis: the symbol of their Initiation into the Sun-Mysteries is the picture of the Sun in Taurus. When the Sun stands in the sign of Taurus the spectacle presented in the firmament reveals the mystery of the particular initiation of the Rishis. This initiation took effect through the seven personalities who were the seven holy Rishis. This is also expressed in the fact that the Pleiades, a cluster of seven stars, shine from the same region of the heavens. That is moreover the region where the whole solar system entered into the universe to which we belong. So in order to specify the various forms of initiation into the Sun-Mysteries we can use expressions indicating the Sun's position in a particular constellation.
But at the same time he also mentions how people ‘invent’ all kind of stories
You will have heard of a certain savant's ironical attempt to establish that Napoleon was not an historical figure. The argument was that the name ‘Napoleon’ is easily derived from ‘Apollo’, the prefix N indicating comparative rank—therefore a kind of super-Apollo. Napoleon had six brothers and sisters and the star Apollo is included among the seven Pleiades. Napoleon's twelve Marshals are said to be the twelve signs of the Zodiac and Apollo's mother, Leto, becomes Napoleon's mother, Letitia
... and so on, in the same strain!
The story of Pleiadian extraterrestial beings
The stories about Pleiadians as extraterrestial beings appears to find its origin in 20th century stories by Billy Meier (1937-) in the 1970s.
(1) He claimed to have been in direct contact with these beings who were benevolent and spiritually advanced and wanted to help humanity avoid self destruction through moral decay, wars, environmental collapse.
(2) He described these beings as tall, blond, human-like and came up with various materials (photos, videos, etc) that however were widely criticised as hoaxes. Note: the blond humanoids or Nordic alien archetype may have been inspired by earlier descriptions by ‘experiencers’ (a.k.a UFO-abductees, see more below) George Adamski (1891-1965) (and others, such as Howard Menger (1922-2009) who claimed they were Venusians).
(3) The Pleiadians were descendants from the Lyra star system, and shared a cosmic or galactic history and philosophy (It seems Meier later corrected this to another star system called Plejaren, in another dimension of spacetime).
The story seems to have been picked up by the new age movement since the 1970s to present, sometimes mixed with other storylines about ascended masters or other guides of human evolution, reptilians and other negative alien forces, etc.
In the 1980s and 1990s, various authors produced so called-channeled texts claiming to come from Pleiadian entities. Authors such as (maybe the most well known) Barbara Marciniak (1948-), but also a.o. Christine Day, Lyssa Royal, etc. At the same time Meier’s stories themselves were 'studied', re-hashed and marketed by various authors, eg Randolph Winters and others.
Over the decades the extraterrestrial beings were sometimes linked to Sirius, Lyra, Pleiades and more .. in stories about ‘galactic’ history, etc. The stories also have a materialistic colouring with physical contacts and forms, DNA activation, etc.
So over the years numerous books have appeared as authors have made a business out of this, sometimes mixing it with whatever additional personal thoughts or opinions and conspiracy stories.
Our contemporary worldview based on materialistic mineral science simple says all these stories are are a fraud or hoax, delusional with no basis in reality. And I would say we can agree they are 'misguided'.
Smoke and fire
However, taking a step back .. if we do not get stuck or fall over the letter but focus on the spirit and the core themes in the narrative, these beings are
(1) Linked with human origins and evolution: they seeded life on Earth and/or helped early humanity evolve
(2) Linked to guiding spiritual evolution of humanity to brotherhood and higher states of consciousness
(3) Consistently bring warning messages: about war, materialism, pollution, technology misuse.
Now interestingly enough these are indeed exactly the main themes that come out of over fifty years of research on what is unfortunately called ‘UFO abductions’ but ought to be treated as ‘experiences’, hence the term by John Mack ‘experiencers’.
If we make this link, things become more clear, see Advanced spiritual beings joining Earth development since last century. The topic is definitely worth study, but because this is a long topic page one can start by just looking at Schema FMC00.650.
The more one realizes the future of humanity, where we are heading, and one studies the conditions that can be expected in the millenia to come with the Reunion of Moon (and upto Reunion of Sun), the more the pieces of the puzzle fit together.
Conclusion
Further research shows that indeed the general view is that Billy Meier was influenced by the stories of early ‘experiencers’ (a.k.a UFO-abductees) in the 1950s and 1960s, even if these never mentioned the Pleiades.
And so one can distinguish (a) a core essence, and then (b) the form and language in which it is ‘packed’, and what various individuals make of it when they ‘bake their own cake’ version of it, like was the case with Meier and those after him. Elements of the essence (a) can indeed be found in more reliable truthful sources; however the whole Pleiadian myth or religion that developed (b) is an unfortunate sidetrack for those lacking further discernment.
In this context I often refer to this short paragraph by Owen Barfield, ever more relevant in times of infoglut and desinformation, and tests for our spiritual discernment of truth.
Owen Barfield: 'Anthroposophy and the Future'
Hope this answers your main question, but don't hesitate if you have add-on questions or want to go into any aspect.
PS: This question reminded me of another one asked on this forum before, about stories that can be found on internet forums about the archons, see Topic:Xy0q2cp1ogfeke8v
