I thought the deception in the opening of the second sentence of the preamble was a cheap trick of the kind I did not expect from Shackleton. It was clearly a deliberate trick because the sentence is a very unnatural and unidiomatic way of saying what was meant. I wasted time trying to interpret it at face value and it just irritated me.
I've abandoned the puzzle (very rare for me) because I cannot make any thematic sense of the the three entries referred to in the third message. I've considered them literally and as cryptically coded items, but their relevance and commonality eludes me. I have an inkling of the theme but have found nothing completely coherent in the grid. After ages spent on internet searches and on grid-staring it's time to admit defeat and release myself from the torture.
I should add that I'm not asking for help.