Okay, the finale. As I said in chat, I watched a lot of fireworks, boosting noncombats and walking away from choice encounters. I eventually had the idea to try to match the fireworks I was launching to the fireworks I had just seen, immediately launching one after the encounter: For the two sparkling fireworks, I launched gold. For the triple fireworks I launched roman candles, for the pair of fireworks I launched twins, and for the single I launched dangerous. But I also messed around a bit earlier and I don't know for certain what else I did. I know I launched a gold for the triple one at least once. I'd also killed a number of fireworks serpents, and attempted to beat the twin serpents a few times, not very effectively. I also got all the roman candles available from the guy on the quad.
I did not launch fireworks after every firework display I saw, and I only launched on the docks. After Kinak fixed the encounter that announces the fireworks are starting, my next fireworks encounter was the finale. So maybe I had triggered it earlier but just couldn't get it until I'd had that first adventure? I don't know.