I enjoyed this as an alternative to the usual gimmick of simply restoring a missing letter. The latter to me often feels like "try all 26 letters in all N spaces of the word until you get one that sort of means what you want it to mean". It was nice to have an alternate means of finding those letters through grid entries.
Can't imagine how challenging this was to set. Requiring most of the grid entries in standard order to have the required one letter substitutes must have been a challenge.
Thanks for the fun eXternal.