Nice approach. It reminds me of an approach I saw used to resolve coordinates to countries. Instead of loading all country polygons, the team created a bitmap and used colors to map each pixel to a country code. The bitmap wasn't super large and compresses pretty nicely in png format. This worked well enough and it dumbed down the country lookup to simply figuring out the color for a coordinate. Neat trick. And you could probably figure out if you are dealing with an edge case by simply looking at neighboring pixels and fall back to something more expensive if you hit one of those.

And of course with edge cases, there are lots of them but mostly it's fine. One case that comes to mind is that of the border town of Baarle-Nassau On the border with the Netherlands and Belgium. This village has some of the weirdest borders in the world. There are Belgian exclaves inside Dutch enclaves. In some cases the border runs through houses and you can enter in one country and leave in another. Some of the exclaves are just a few meters. There are a few more examples like this around the world.

Another issue is the fractal nature of polygons. I once found a polygon for New Zealand that was around 200MB that broke my attempts to index it. This doesn't matter of course for resolving country codes because it is an island. But it's a reason I implemented the Douglas Peucker algorithm to simplify the polygon mentioned in the article at some point.