As JF said, your thoughts on Canto Four have sparked a lot of interest. Kudos to you for an engaging discussion. I think you may be overselling the degree to which the lack of art reveals madness. As Jerry, I believe, pointed out, it may be that Shade just didn't have a chance to revise, though it had been his practice to edit as he went along. Yet I do think there is a bit of unraveling there which, in combination with many other details from the novel as a whole, may reveal an ebb of sense. Actually, I think you give Shade too much credit for the envoi, as you call it. While lines 963-976 are striking and meaningful, the rest is a jumble of odd details.
I'm reasonably sure that we survive
And that my darling somewhere is alive,
As I am reasonably sure that I
Shall wake at six tomorrow, on July
The twenty-second, nineteen fifty-nine,
And that the day will probably be fine;
So this alarm clock let me set myself,
Yawn, and put back Shade's "Poems" on their shelf.
I sometimes read the opinion that these lines confirm Shade's confidence in the afterlife. But look at all the qualifiers! How sure is he? Reasonably sure. What does reasonable mean? It means that he is as certain of Hazel's existence as he is that he will wake up tomorrow (okay so far) "And that the day will probably be fine." So the afterlife is as certain as the probability of good weather--that is to say, not very certain at all! The word "probably" is a significant hedge. So after that less than definitive declaration, the culmination of the whole poem, we get two flabby lines describing the speaker preparing to wrap up his day (and his poem). Thud.
Though these lines hardly inspire, they might have served as a logical ending for the poem, but Shade needs to get us to line 1000, so we need another 15 lines. In these we find:
1. A description of Old Dr. Sutton's windows reflecting the sun.
2. Idle conjecture about Dr. Sutton's age.
3. Shift to wondering where Sybil is.
4. Acknowledgment that Sybil is near the shagbark.
5. Sound of horseshoes & mildly clever metaphor picturing leaning horseshoe
6. Lovely description of dark Vanessa
7. Unknown man trundling wheelbarrow
8. Presumably a repetition of the first line.
Again, many, if not all, of these details end up being important to the novel as a whole, but they are trivial in the poem itself. Readers have no cause to care about Dr. Sutton's age, or to care about John Shade's interest in Dr. Sutton's age. The Sybil/shagbark conjunction reminds us of Sybil's grief re: Hazel, which is fine, and the dark Vanessa has already been associated with Sybil, but these loosely associative images don't really take us anywhere new; nor do they serve as an effective denouement. I suppose the twilight gives everything a kind of purple, melancholy patina? But then we get the final interruption--a man with a wheelbarrow. Who is he? What is he doing in the penultimate line of the poem? And how does that wheelbarrow lead us back to line 1? I know the relationship to the clockwork toy, but I don't think Shade himself is making that connection here. As he said earlier, it really does seem like his brain is drained and he is no longer able to determine which images have value and which do not--everything is equivalent: alarm clock, poems, Sutton, Sybil, horseshoes, butterfly, wheelbarrow, [waxwing].