I think this crossword will divide solvers into two camps: those who think the preamble should be clearly written to assist solvers; and those that think deciphering the instructions is another worthy and justified layer of obstruction.
The first sentence starting in the seventh row of the preamble is applied to determine the letter represented by 'a' - if b, c, d and e are known, there are two candidates, one likely. The external cells confirm which one it is.
The solver then must work out which [clue] number corresponds to this particular letter, by a brute force, trial and error approach - I think it's a stretch to say the value can be 'deduced'.
As such, I suspect many are in your camp, of a completed grid and then frustrating endgame - though it is unambiguous once solved.