What about the fourth person posited by William Dowling (full disclosure: my professor at the University of New Mexico many years ago, so I have a bias). The short story is that there's no other explanation for this passage, which has parenthetical injections from many characters and research that Kinbote claims he was unable to do.
He began with the day's copy of The New York Times. His lips moving like wrestling worms, he read about all kinds of things. . . . The United States was about to launch its first atom-driven merchant ship (just to annoy the Ruskers, of course, J.G.) Last night in Newark, an apartment house at 555 South Street was hit by a thunderbolt that smashed a TV set and injured two people watching an actress lost in a violent studio storm (those tormented spirits are terrible! C.X.K. teste J.S.). The Rachel Jewelry Company in Brooklyn advertised in agate type for a jewelry polisher who "must have experience in costume jewelry" (oh, Degre had!). The Helman brothers said they had assisted in the negotiations for the placement of a sizable note: $11,000,000, Decker Glass Manufacturing Company, Inc., note due July 1, 1979, and Gradus, grown young again, reread this twice, with the background gray thought, perhaps, that he would be sixty-four four days after that (no comment). . . . A pro-Red revolt had erupted in Iraq. Asked about the Soviet exhibition at the New York Coliseum, Carl Sandburg, a poet, replied, and I quote, "They make their appeal on the highest of intellectual levels." A hack reviewer of new books for tourists, reviewing his own tour through Norway, said that the fjords were too famous to need (his) description, and that all Scandinavians loved flowers. And at a picnic for international children a Zemblan moppet cried to her Japanese friend: Ufgut, ufgut, velkam ut Semberland!" (Adieu, adieu, till we meet in Zembla!) I confess it has been a wonderful game--this looking up in the WUL of various ephemerides over the shadow of a padded shoulder.
The full screed: http://www.rci.rutgers.edu/~wcd/palenarr.htm
To me this suggests, convincingly, that an author-character has deliberately invaded his own creation at this point, much as one did at the end of Bend Sinister, and so there's hardly any point in imagining any of the characters as anything but a figment of HIS imagination. Though of course we can continue to argue about what that author-character constructed before the intrusion, but I think there's a clearer and more literal interpretation -- K=K, S=S, G = figment of K's imagination.
--Tim Henderson