Here Pollux is certainly not the father of Saturn. He is a sun of Jupiter and a "symbol of destruction," whose name he derives from the Greek meaning "seeking to destroy." Allegorically, he and his siblings are the inevitable result when magnanimity descends to insults. Anyone who read this far would not mix up Pollus and Pollux.
This objection is not conclusive, because after all Ridewall did mix them up, explicitly. It is merely a problem. It is possible that someone devised an explanation for how Pollus and Pollux really are identical (e.g. reincarnation, as in the Orphic account of how Phanes and Dionysus are identical); but if so I have not seen it.
Also, there is the difficulty which you recognize, Huck, that the 15th century humanists had many other texts at their disposal, in which the father of Saturn is never either Pollus or Pollux.
Reading Martianus Capella, however, I see another solution to the problem of why the sequence of the signs of the zodiac in Minchiate ends in Gemini. It is two paragraphs further in The Marriage of Philoology and Mercury than the account of the Muses and their assignment to the various levels, from earth (Thalia) to the "outermost sphere" (Urania) (this is at 1.27). Having seen the Muses, Phoebus (Apollo) and the Cyllenian (Mercury), journeying upwards to seek Jove's permission for Mercury to marry Philology, are suddenly transformed into planets, Apollo into "the guise of the sun" and Mercury, too, "into a glimmering planet" (both 1.28). The next paragraph (1.29) then states:
Thus enhanced by this transformation into celestial bodies, they were carried through Gemini, a sign kindred to them, and they shone forth in the majestic sky and soon sought the palace of the Thunderer. (Stahl and Johnson translation, p. 17).
Gemini is kindred to them because the Gemini (Pollux by birth and Castor by either birth or Pollux's sacrifice) have the same father as Apollo and Mercury. But more importantly, it is one of the two signs ruled by Mercury. What is being brought out, I think, is the idea that to enter heaven, the place of Jove, the gate is at Gemini, because Gemini has a special relationship to Mercury. (He could perhaps also have used Virgo.) Mercury is the god who escorts souls to heaven, as we know not only from this book, where he escorts Philology there, but also from Apuleius's "Cupid and Psyche," where Psyche is allegorically the soul. In the Minchiate, the signs of the zodiac occur between the four elements and the unnumbered cards of heaven. They are in the 8th sphere, which separates the cosmos from what is above the cosmos. The Star card, which comes next, is not about a star in the cosmos, but rather Christ as symbolized by a star that transcends the stars and fate (iconographically the Star of Bethlehem on the Minchiate Star card, or the star on the Hope card). To enter the realm of Christ, then--if one follows Capella--one passes last through the gate of Gemini.
I notice that in the Minchiate the signs of the zodiac are all in a very peculiar order, which I do not recognize. So I would feel better about this solution, or any solution, if I knew why in Minchiate that order as a whole was chosen, from Libra to Gemini.
.... I'm also not so sure, that the Taurus-Gemini connection is really "a sign". But somehow, the Castor-Pollux story seems to be "specific fo Florence" in 16th century and possibly later - as if the Medici had an identification. It just jumped into my eyes.