I totally agree with you about the author's voice/opinion first occurring in line 18. I'm going to email our book people about that to see if they want to tweak that wording for the next printing. Thanks!
Frequently, seeing a sentence prefaced by "it can be argued" would mean the author is bringing up a point of view, but not necessarily one that she agrees with. Sometimes authors just 'present the debate'. And so I think the book writers were probably saying that AT THAT POINT in the passage, we're not sure if lines 18-27 are really the author's thoughts or whether the author is just presenting common rationales people use to debate these critics.
By the end of the passage, we clearly see that our author IS involved in arguing against these critics, so I think it's fair, in a revisionist way to say that lines 18-27 were the author talking.
(Was that lawyerly or what? I just defended my client, whether or not I believe in the merits of his case)
In terms of institutional 'consensus' vs. 'authority', I think the passage was pretty much using them interchangeably. In order for institutions to exert the power of their arguments on people, the institutions would first have to agree to a certain law/standard.
So we could say a maxim such as "thou shalt pay your income tax" does not have intellectual authority ... it's not necessarily a claim that someone intellectually must believe. But since the government is set up with laws that say you have to pay your income tax, we would say that there is institutional consensus on the matter, and hence institutional authority behind that maxim.
Meanwhile, a line like "intellectual authority and institutional consensus are not the same thing" refers to things like Galileo advancing the argument that the Earth revolves around the Sun, not the other way around.
Galileo's argument has intellectual authority, while the geocentric (earth-centered) model of the world had institutional consensus/authority in Galileo's time.
The idea in P3 is that if there's consensus within the world of musicology that "5 decades is long enough to judge whether an artist was influential", then you can derive statements with intellectual authority by using that arbitrary (institutional) standard.
If Johnny 2-Sox's experimental harmonica ballads didn't find any significant audience within their first 5 decades, then everyone can intellectually agree that he was not an influential artist. The critics are saying, "How arbitrary that you concoct this 5-decade measuring standard! That standard has no intellectual authority, even if the judgments you render by that standard are agreed to be accurate according to that standard."
The author agrees with this, but says that judges have the power to "change the standard" (ditch the somewhat arbitrary precedent). The fact that they choose to do so when the precedent seems to be a poor fit for society/reality shows that judges are using intellectual considerations to render their judgments, not just institutional ones.
Hope this helps.