Happy with all the parsing, finally, but a little concerned about a few points.
(i) No-one has mentioned this, but the last word of the 13 letter restriction from the play (when the one unused letter is added) seems to me to be the wrong spelling for the endgame to work. Is there justification in the BRB?
(ii) Does the completed restriction from the play replace the ellipsis in the title of the puzzle? (Just thought of this!)
(iii) The final grid has a symmetrical pattern resembling a letter. Is this significant to the theme or am I overthinking this?
Great puzzle though, Demoniacal — first of many?