Dryden, don't give up!
1. You are right, but you're only half way to solving the unclued entry. Unfairly, perhaps, a substitution *is* required in *one* of the intersecting across clues.
2. Where a grid entry appears to have either limited or no potential for substitution, you need to keep in mind *not only* that two entries remain unchanged *but also* that the preamble doesn't say that substitutions will only occur in intersecting cells. It also doesn't say that clashes, where they do occur, must be resolved in favour of one of the two that initially appear!
3. The 'shortened' entries aren't shortened.
This is a brilliantly constructed puzzle that I thought at first was unsolvable. The only way I got there was to write out all the (apparently) possible clashing *and* non-clashing possibilities and stare at them for a very long time.
The required pairs do indeed read non-consecutively, but my breakthrough came from studying the downs first; I've given all the other thematic hints I can, short of spelling everything out, in my earlier posts.
Good luck!