Also concerning number 1, I found a citation that mentions "painted boards" that the Incas stored in a library and appointed men to interpret and read. Sariemento Gamboa, after consulting as assembly of Inca historians, recorded the following in reference to the Inca Pachacutec Yupanqui:

"…after he [Pachacutec] had well ascertained the most notable of their ancient histories, he had it all painted on large boards, and he placed them in the house of the sun, where the said boards, which were garnished with gold, would be stored as in our libraries, and he appointed learned men who could understand and explain them…" (source, pg 107 of the first link)

Apparently Molina and Cieza de Leon also mention these boards in their own Spanish chronicles. But where did Knorozov get the specific detail that they were specifically written in reverse boustrophedon???

Current academic thought on contact between South America and Polynesia amounts to "Yeah, sure, why not." These two comments from /u/uncagedbeast and /u/b1uepenguin go into more detail. The persistent issue is that evidence points to the results of interaction between the region, but there is yet no evidence of that interaction itself. Did folks make it from Polynesia to South America and back? Probably. Can we say anything beyond that? At this point, not really.

There's not too much out there regarding Yupanqui's voyages, mostly because there is very little to go off of. Sarmiento de Gamboa does tell us a little more than what is quoted on Wikipedia. Topa Inca Yupanqui was visited by some merchants from the islands, they spoke of gold, Topa Inca asks his advisor Antarqui if they're telling the truth, Antarqui trusts them, Topa Inca creates a fleet and loads his general onto the boats, returns 9-12 months later with gold and bones and leather, and executes the general he'd left in charge who seemed a little too happy Topa Inca was gone. Sarmiento remarks that his story might seem odd but that the guy who told him the story used to own the bone and leather that Topa Inca brought back.

At its core, there's very little to this story. Much more time is dedicated to the politics of the trip (which generals went, the decision to trust the foreigners, etc.) than is to any substantial details of the trip itself. Likewise, Sarmiento de Gamboa's source has big "I found a cool bone and made up a story about it" vibes. Those who think the voyages happened often cite corroborating accounts from two other chroniclers, both of whom were working from the same, now-lost intermediate source. On the other hand, Juan de Betanzos wrote a chronological narrative of Topa Inca's exploits that generally parallels Sarmiento de Gamboa's, yet excludes this event and leaves no room for it in the timeline. Most important, however, is the fact that other stories of exotic visitors from the Pacific pre-dated Topa Inca himself. I've written here about the legend of "Ñaimlap," the supposed mythical founder of the Lambayeque kingdom. The legend is drenched in themes, objects, and characters particular to Andean society, to the extent that the far-off origin of its protagonists feels more like an intentionally vague, un-falsifiable obfuscation than a recounting of real events.

But let's get to your title question: what is this writing that Butinov and Knorozov refer to?

The "well-known Bolivian scientist" they mention is moderately-infamous Argentinian writer Dick Edgar Ibarra Grasso, and they are undoubtedly referring to his 1953 publication La escritura indígena andina, an influential study of a set of documents from Bolivia and southern Peru that can be broadly classified as "pictographic catechisms."

The pictographic catechism is a fascinating "genre" of document which uses lines of stylized figures to represent common Catholic prayers. Most specimens contain the Ten Commandments, Our Father, and Confession; longer examples may include the Apostle's Creed, Articles of Faith, and lists of the Seven Deadly Sins, Works of Mercy, or Virtues. If you pull up the text to the Creed, you can follow along with the particularly well-preserved Huntington Catechism: heaven and earth are circles with God and a church inside, Jesus died and was buried, and it ends with everlasting life.

Several dozen of these documents can be found in museum and library collections, with the largest collection being that donated to the Universidad Mayor San Simon in Cochabamba, Bolivia by Ibarra Grasso. "Classic" examples were written on leather using the juice of the ñuñumayu plant, while others appear in standard-issue school journals or on repurposed newspapers, ledgers, or customs documents. Some of these are written in the boustrophedon format; many, like the Huntington above, were not. I cannot find any historic examples of reverse boustrophedon in which figures are rotated rather than mirrored. This would be fairly easy to spot, given the number of people and crosses who orientation is obvious.

The earliest reference to such a document comes from Swiss diplomat and naturalist Johann Jakob von Tschudi who traveled the region in 1830s-50s. While arriving in Bolivia, he stopped in the town of Sampaya on the shores of Lake Titicaca and was shown a sheepskin with rows of figures by the local priest, who had it read by a local girl:

By order of Father Areche, the girl rapidly read to me the hieroglyphs in Aymara, which contained the Small Catechism. Father Areche told me that an old Indian from Sampaya who didn't know how to read or write Spanish had invented the signs and had painted them on skins and paper; for the writing instrument he used a round stick and the juice of a plant that grows a lot near Copacabana.

Tschudi notes that the lines were read alternating left to right and right to left and claims to have met the man, Juan de Dios Apasa, who invented the symbols.

The British traveler William Bollaert, who seems to keep popping in my recent answers, wrote that he was able to see a similar (probably not the same) object in a museum in La Paz, Bolivia, but was told that the writing system was "ancient." Decades later, in 1911, writer Franz Tamayo published a preliminary translation of this piece, which inspired Arthur Posnansky to include a reproduction of it in his moderately popular guide to the archaeology of the region. You'll note that below his attempts at translating individual symbols he's included some Rongorongo for comparison.

in 1940, Posnansky passed a journal he had found near Copacabana to the visiting Dick Edgar Ibarra Grasso. This sparked the writer's interest, and Ibarra Grasso spent the next decade compiling examples, eventually publishing the 1953 volume cited in the article that got this whole thing started.

So... what can we really say about these "texts?" Two questions dominate discussion: is it writing, and is it pre-Hispanic?