That's a bit explicit, bellabee.
I'm not sure that starting off with the 6 clashes helps with the finale, though I can see how it might. All that's needed is to complete a grid with the 18 clashes clearly filled in, then two thematic elements stand out clearly (well, they did to me), and the rest follows from there.
I didn't like forcing the 18 either, and on my reckoning there are two exceptions to the alternating across/down choices, though I need to double-check that. There also appears to be an anomaly at the end. To my mind the theme does not fall neatly into place, but does have to be forced.