I've no idea how the setter created such a puzzle - thanks, Pandiculator!
I would call this an excellent example of a difficult but clear (and therefore fair) endgame. I much prefer a complex set of instructions to a GWIT. Although there were a couple of interpretations possible (the first cell of the message; each letter on the path, or those stepped upon; 'previously visited' by which path?), the endgame could be solved by plain and fair means. Which does differ to some recent endgames, of course not without their own merit.
I am astonished nobody has commented on the 'false path' laid in the middle column. I re-read the preamble many times and have settled on the letter completing the second entry to be highlighted, but my god, a second letter is tempting!
An excellent Listener - thanks Pandiculator!