I am not in a position to answer your question about scholarly consensus regarding the two symbolic meanings of the choice of the Piraeus. There is, however, a discussion of the framing story of the Republic that I encountered recently and found fascinating. That discussion addresses what was said to be going on in the Piraeus just before the dialogue, rather than the port location as a location with characteristics and a history.
The discussion is in the concluding chapter of Religion in Human Evolution: From the Paleolithic to the Axial Age (Cambridge, Massachusetts and London, England: The Belknap Press of Harvard University Press, 2011), by Robert N. Bellah. It is part of a longer discussion of the evolution of the term theoria and its successor, “theory”. Bellah in turn draws on Andrea Nightingale, Spectacles of Truth in Classical Greek Philosophy: Theoria in Its Cultural Context (Cambridge: Cambridge University Press, 2004).
Bellah writes (on page 577):
She notes that Plato himself begins and ends the Republic with examples of traditional theoria. The dialogue begins with Socrates going to the Piraeus, the port of Athens, to attend the festival of the Thracian goddess, Bendis, suggesting that the festival was more “international” than the short distance to the Piraeus might indicate, especially in view of Socrates's remark that the Thracian procession was as fine as the Athenian one, expressing a Panhellenic viewpoint. And the Republic ends with the Myth of Er, which turns out to be a most remarkable theoria, because Er, who had been killed in battle and was about to be cremated, awoke and told his fellow countrymen about a journey he had made to the land of the dead and the festival he had attended there. [Bellah here cites pages 74–77 of Nightingale.]
Thus, on Bellah's recounting of Nightingale's observation, it is the religious festival taking place in the Piraeus, and Socrates's role as a theoros of that festival, that is salient as the opening of the narrative frame for the Republic.