Artistically speaking, I think Sanborn handled the cryptography like any other medium and used just enough to be dangerous methods to transform a message into a fully realized piece of art, utilizing a past and present context in an expression of the self in relationship with the world.
I think I made a mistake to think there was some greater purpose in using a calendar as a palimpsest. It's reasonable to consider he rendered K1's message in such a way as to make the misspelling of IQULUSION take on the guise of a calendar date especially with K3's being a fit for the 26th day of the month. At first it seems like a great discovery, but in thinking beyond, it doesn't really drive the puzzle forward. It wasn't really necessary to determin the origin of K3's message. It doesn't do anything unique or satisfying to think the position of the misspelling in K2 is an indication of K4's message. I think I was just wrong to think it mattered so much.
I may have discovered a more practical use for placing intentional mistakes in the message. They trigger a length/position reference. IQLUSION's Q is at position 57. The midpoint of the message from the first letter to the last letter just before the Q is the 28th position, right on the B of ABS, and that's no accident since a dictionary lookup of ABS can put you right on top of the next message's keyword: ABSCISSA, which solves K2.
When I apply the same observation and technique to K2, I discover that the U of UNDERGRUUND is positioned at 116, and that places one the absolute center of the message from beginning to U as 57.5, the spot between letters AG. I believe the AG is our hint for a keyword applying to K4.
B in the 28th position of K1 is also the name for Element 6, Boron.
Ag in the 57th position of K2 is Element 47, Silver.
It doesn't feel like a puzzle about the periodic table of the elements, so I'm just covering my bases.
Something that has me curious is why does K1 exist at all? K1 and K2 are both using the same coding system. Why have us repeat our efforts? K2's location of the misspelling of an O with an U at position 116 may be applicable to the solution process for K3. The U is the 24th letter on that line with 8 letters to follow. K3 is solvable by fixing it to 24 columns, turning, and then fixing the result in 8 columns and another turn. If K2's misspelling hints at 24/8, then maybe K1s misspelling hints for a double transposition of 25 and 6.
I've been thinking about how K0, K1, K2 and K3 are solved in succession, and then K3 gives us a passphrase hint XCANYOUSEEANYTHINGQ. I'm imagining a process that takes K1 and K2, 436 characters all together, and double transposes it by 25/6 (from K1). I think a playfair comes into play at this stage. We have the subtle hint of an X, although a standard in telegraphy as a stop, it is also common to see it inserted in cases of double letters in the playfair cipher. A Q would be added to the end of the message to make it even.