It seems to me that one might summarize Pale Fire, the poem, as follows:

Canto 1:  Shade's early life,

Canto 2:  eschatological commitment, Maud's stroke & commitment,  Hazel's story

Canto 3:  Shade's grief and attempts to heal and find meaning to, or in, life. (IPH, and the White Fountain)

Canto 4: Proclamation of great insight, 

description of two methods of composition, one of which is compulsive, 

travails of shaving,

epilogue or envoy.

Shade's hopes for an understanding about the hereafter, thence existence, and the meaning of Hazel's life and death in his vision of the White Fountain. This proves illusory and is succeeded by the theory of plexed artistry, which is declared at first boldly, and then tentatively at the end of Canto 3. The reader at the start of Canto 4 is thus prepared to hear more about plexed artistry, the web of sense, and how Shade finally comes to terms with his daughter's death. 

Shade begins the final canto with great resolve, promising great insight. The lines are though a grotesque overstatement of resolve. The grandiosity is itself symptomatic of mental illness. The belief that one possesses insights unavailable to others is both grandiose and pathological. It also links Shade directly to Kinbote in that grandiosity is Kinbote's chief defining attribute. The reader is seeing Shade mutate into Kinbote right before his eyes.

Immediately after making his grand pronouncement, Shade loses his train of thought. Another sign of mental distress. He links to the preamble in a way that is notable for its clumsiness, And speaking of this wonderful machine, and its sudden shift of affect, from grandiose resolve to a desultory questioning. This shift of tone tends to confirms that Shade is mentally not well. These abrupt affective shifts recur throughout the canto, especially during the shaving sequence. 

If one accepts that Shade has lost his train of thought right after his preposterous preamble then the reader may well wonder why, given that this is a poem, and that poems are usually creatures of revision, why doesn't Shade just revise and smooth things out? Partly he knows his time is slipping away fast and there is no longer any time for editing. But partly too because he is changing into Kinbote. Kinbote has said he's no good at versifying and the reason is simple: he, like so many other writers, (preeminently Jack Kerouac,) never revises. (I'm not going to support this last point [about Kinbote's aversion to revision] right now, but it is supportable and hope that the reader accepts it. Let me know otherwise.)

The fable of Shade's shoe, and the sense of triumph he derives from it, are also symptomatic of magical thinking. 

Shade's continuation after his exposition on versification, his shaving travails, is another non-sequitur. Why would his reader want to know, in detail, Shade's shaving habits? Its purpose is to show Shade's derangement through his emotional and compositional absorption in mundane trivialities. Shade's litany of loathes is the climax of this nonsense as well as the dramatic climax of the poem, the symbolic collapse of Shade's persona, ( he still survives to write the envoy), and the fulfillment, by way of explication and analogy, of the abstruse waxwing metaphor that begins the poem. 

If the reader does not believe that John Shade is crazy, then the line, and a brown ament ... lie on the cement, must offer no shade of foreshadowing i.e without ament's second meaning of a demented individual one is left only with a catkin on the cement; what's the significance of that? 

Also the other half of the two-fold metaphor the noun I meant to use, dry on the cement, how is this to be decoded? That an index card of unused key words has fallen out of the pack and lies on the sidewalk, (which technically I'm told is not cement but concrete.) Can cement be interpreted here metaphorically? My point here is that Shade's metaphors have become exceedingly abstruse, indicative of looseness of association.

Shade's litany of loathes rages out of the same imbalance: random, unprepared and trivial.

Shade's shaving routine provides the trivial material needed to depict his descent into the madness that is Kinbote. 

Engagingly yours, I hope,

ps: and now it's time to shave, and fondle my rubber duckie!