The preamble had a very long series of steps: misprints leading to “hints” leading to “questions” leading to “elements” leading to “another message” which then had be applied to the grid. It all seemed very confusing to start with, but the first step was clear enough: “Most clues contain a misprint of one letter in the definition.”
It took me about four hours to fill the grid, with some very tricky misprints to find.
19 across “Perhaps referring to stroke in eighty year old, retired—ultimately monoplegia’s assumed” yielded OARY (all letters being checked, so no doubt about it), but I fell into a trap when I spotted that “eighty” ⇒ R. In fact the misprint was “eighty” for “eights”, and the wordplay was “year” ⇒ YR “old” ⇒ O, reversed around “ultimately monoplegia” ⇒ A.
46 across “Pretence one found in king and queen?” looked like it had the wordplay “one” ⇒ I in “king and queen?” ⇒ MEN yielding MIEN, but where was the misprint? Perhaps “pretence” for “pretense”, but I wasn’t sure I could justify “pretense” ⇒ MIEN from Chambers, which gives “pretense … show or semblance” and “mien … air or look, manner, bearing”. Are these close enough?
28 down “Yankee hosted by queen—Earl shed some tears for her” took me a long time. The wordplay was “yankee” ⇒ Y in “queen” ⇒ RANEE from which “Earl” ⇒ E has been removed. The misprint was “tears” for “years”, and the definition was “some years for her [i.e. queen]” ⇒ RAYNE.
The clues without misprints seemed in some cases to be rather oddly worded, with imprecise definitions or extraneous words, as if they had been written under rather severe constraints.
The corrected misprints yielded two “hints for deriving two versions of a question from the remaining clues”: DOT ONE’S ‘I’S AND CROSS ONE’S ‘T’S in the across clues, and IDDY-UMPTY in the down clues. Chambers: “iddy-umpty n Morse code. [From a phrase used in India to teach morse to the native troops].”
Interpreting I as a dot, and T as a dash, the remaining clues could be treated as Morse code letters: this yielded MUST IT BE? and MUSS ES SEIN? The latter question (of which the first is a translation) appears in the manuscript of Beethoven’s String Quartet number 16 in F major.
Third step: “The penultimate element of the first version is one of four that share identical components, as described by a five-letter definition concealed in the grid.” Taking “element” to be a letter, the “penultimate element of the first version” is the letter B, which is — · · · in Morse code. Taking “component” to be a dot or a dash, there are three other letters that are encoded with the same set of components: · — · · is L; · · — · is F; and · · · — is V. The five-letter word concealed in the grid was PAEON, “a foot of four syllables, any one long, three short”.
Fourth step: “These four elements and another five-letter word must be highlighted to show key information relating to the question and an initial representation of the questioner.” It was notable now that the four letters B, F, L, and V appear once each in the grid. The “questioner” was L[udwig] V[an] B[eethoven], and the “key information” was F MAJOR.
Fifth step: “One of the four elements, interpreted differently, indicates which letter of which word in each non-misprint clue contributes to another message.” (This explains why some of the non-misprinted clues seemed rather awkward.) Well, the element that could most obviously be interpreted as a number was V. And the fifth letters of the fifth words of the non-misprinted clues spelled out DIAMETRIC EXCHANGES.
Sixth step: “The action it describes must be applied to the letters of the five-letter definition [that is, PAEON] (and their counterparts) to reveal a representation of that element which must be highlighted in full.” Swapping PAEON in the fourth row with IDTID in the tenth yielded DIT DIT DIT DAH, a representation of the Morse code · · · — for V.
The preamble promised that “all entries are words in both the initial and final grids,” and sure enough the diametric exchanges yielded new words: PYNE → DYNE, SQUARE → SQUIRE, PHAEIC → PHATIC, OLIO → OLID, TINNER → TINIER, PAINED → PANNED, DOSE → OOSE, OCTANS → OCEANS, THINKS → THANKS and DEED → DEEP.
Seventh (and last!) step: “Finally, the key information must be modified to provide a consistent rendition.” Now, DIT DIT DIT DAH is not only the Morse code for V, but a representation of the opening motif of Beethoven’s Symphony number 5 in C minor. And sure enough, changing F MAJOR to C MINOR yielded new words: FONE → CONE, TACKY → TICKY, and A MAJORI → A MINORI.
The last movement of Beethoven’s String Quartet number 16 is headed, “Der schwer gefaßte Entschluß” (the difficult resolution). It was quite the reverse in the resolution to this puzzle: the well-written preamble led me very smoothly through the complicated series of messages and transformations. If anything, I was slightly disappointed at how straightforward it all was. But any disappointment is overwhelmed by my awe at the amount of thematic material Shackleton managed to cram into this puzzle.
I’m not sure about the title. Perhaps it’s an answer to the thematic question?