I realize I'm getting to this pretty late, but I only recently evolved from lurking to making an account and tentatively posting. I'm a little confused by what you're asking here, however. Are you asking if the composition of guanine is as portrayed by the text/if the alloy exists?

If so, then the answer is yes, and it was fairly widespread. The Spanish referred to it as "low gold" and there are a ton of names for it throughout various Latin American communities. Most useful reference on it for you, however, is probably the 1501 letter prohibiting the sale of guanin in Hispaniola, which appears in the footnotes of "Africa and the Discovery of America, Volume 2" By Leo Wiener at the bottom of page 117 (link: https://books.google.com/books?id=Uz11AAAAMAAJ&pg=PA117&lpg=PA117&dq=hispaniola+law+1501+guanin&source=bl&ots=tLaO8hqE43&sig=HugM6WiN7YX_Iyf_3RVTIKk8j_4&hl=en&sa=X&ved=0ahUKEwj_w_PslMDYAhUd0IMKHYA_D1kQ6AEIJzAA#v=onepage&q=hispaniola%20law%201501%20guanin&f=false). When you sort through the old Spanish, it basically confirms the existence and states that they don't want it being spread throughout the land/being sold/used, although the book linked does indicate that the metal was still spread secretly. If you want the full text independent from the English analysis, I found it on p.61 of https://babel.hathitrust.org/cgi/pt?id=uc1.b3283733;view=1up;seq=204, which is part of a larger archive of official Spanish documents. Sorry about all the links, but it seemed the easiest way to direct you to specific stuff here.

There's also a fair bit of investigation into the matter in regards to the metallurgical aspect, usually in regards to the guanín possessed by the Taíno, since most of the modern samples of the alloy (all 15 of them) that exist are from Taíno territory in what is now Cuba. Per those samples, it appears that the composition of the alloy does seem to mirror or is at least similar to what Columbus reported. I've included the source for a report on the matter below. It's also interesting to note that there is a Colombian variation of the alloy, tumbaga, which seems to have dropped in importance following the introduction of guanín. I won't profess to be familiar with the metallurgical aspect of the alloy, however, and as such won't do any analysis on it, although there's probably another subreddit (/r/askmetalworkers?) that could help you if you're that interested. I do want to note, however, that if you're looking for further reading, tumbaga gets significantly more hits online than guanin, so that may prove useful, although the two are slightly different in composition and use.

Metallurgy Source lazily formatted: Marcos Martinón-Torresa Roberto Valcárcel Rojas Juanita Sáenz Samper María Filomena Guerra, "Metallic encounters in Cuba: The technology, exchange and meaning of metals before and after Columbus", Journal of Anthropological Archeology, Vol. 31, Issue 4, p. 439-454