I reshuffled the STW tileset, and have the cliff tiles positioned properly to pick up the impassable attribute. I was also able to reposition the 8 terraced rice paddy tiles together without going beyond #169. That does mean extra editing work for me, but I decided it was worth it to keep the tileset as compact as possible.

It's possible that I haven't identified all the tiles which have the impassable attribute, but I think that's unlikely. I've already retextured the first 6 of the 122 STW maps with this new tileset, and the impassable areas on those maps are now correct. I would estimate it will take me less than 2 weeks to fix the remaining 117 maps. Some of those maps won't require any adjustment at all, but I have to look them over carefully.