Soon (tomorrow or maybe even today) a new version of the generator will be released, which will export maps a little differently.
- Possible values of the "terrain" field are: water, rocks, forest-light, forest-dark, forest-dead, desert, swamp, mountain.
- If terrain is not specified for a hex, then it's empty, you can treat it as a plain. I don't see how indicating this explicitly would make anything easier, this would just make files bigger.
- I don't think the current version truncates anything, but it doesn't list any empty water tiles. The next version (the one mentioned before) exports empty water hexes with the terrain type "water".
- I'll think about your "feature class" suggestion. The problem is that while dangers and settlements are special types of features, everything else is just an area and an area can potentially include different types of terrain.