As so often happens, posting (and a night's sleep, being in Sydney) did the trick.
And, of course, once you've seen the relationship it is simple and unambiguous.
I apologise, Elap, for my somewhat grumpy post. It is, indeed, a very clever puzzle. As others have said, finding the clued entries remained a challenge almost until the end. Along the way, the constraint of having to get a sensible message did help.
ByronK, it looks as though you've found the relationship despite my "help".