I have completed this, and followed the instruction from the corrected letters to achieve the satisfying final grid. But although the instruction was obvious, I can't derive it unambiguously from the replacements. In particular, the only possible substitution I can see in "Heathen..." doesn't fit, and I can't see where to use the necessary letter.
It doesn't make any real difference, as I'm sure my final grid is correct. But this annoys me, and I'd like to know!