Thanks foinaven, I now have a full grid and no idea what to do with it.
The clashing cells are revealed as having their length and status in common but Google has nothing helpful to say about them as a group. Nor do I see any pattern with the 15 overruns (someone else said there were 16), any single letters to be used as as replacements, 'internal' letters or a theme.
At least I can console myself with having got this far but - again - it's really all a waste of time. Hats off to the many who have solved it all.