I see Lananok as closer to the Aztecs than the Mayans, and Aztec games were more like war than Aztec wars like games.
Rich merchants can be happy to live on their islands, but they cannot be happy to limit their markets to their islands simply because capitalism requires the fuel of new markets to keep it from collapsing on itself. Rennaisance Venice was an empire. They were a small player in the New World compared to Spain and Portugal not because they weren't interested in colonies, but because they backed the wrong horse against the Ottomans and that limited their resources -- which meant that Spain and Portugal were getting influxes of wealth from their immense colonial efforts.
I could see Sel-Kai not doing much colonization of Lankanok becuase their money's tied up in Stroani and Rhakhaani wars, but with Lankanok being this huge plum right in their back yard I'd think it more likely they were focussed there -- with the surge in Zanarian influence due largely to their being able to fill the Khatra's coffers with coin in the 6030s-40s TE. Even then, there's no way that someone growing as rapidly as Relian is doesn't push through the Choak and take a big bite -- if only to ensure it's got the wealth to be able to avoid being colonized by Stroane.
Pre-WWII America was wildly colonial. The Monroe Doctrine was pretty much the U.S. staking a claim on South America as it's colonial property. They'd have certainly defended against the attack, but I think they'd have dropped moderately sized rocks instead of canister bombs. Revealing the bombs can only result in everyone in the region looking at Namar Tol as an existential treat. Rocks are far safer in the long run, and the tech difference would still result in the joke "Why does the New Lankan Navy have glass-bottomed boats?"
I see a continuity issue with explaining it away with essence flows -- those flows didn't stop Lankanok from sending a navy at Namar Tol. If Lankanok's pre-iron then Sel-Kai, Preten, Danarchis, and pretty much everyone else on the Haestran and Silaari coast is going to have a better navy better able to handle them. Certainly the flows are navigable enough for Sel-Kai to have a stable gold trade with Lankanok.