1. I'm pretty sure there is no way you can expand it once you've made it. What I do from preventing that kind of problem is starting with a 10x10 scenario. If I end up needing something longer, I can use nodes etc. to fit my needs.
2. As far I know, they are in fact unchangeable. I couldn't tell you why, though. Those properties are probably permanently linked to the terrain #.(As a side note, I found out that if you change one of the road terrains to, say, a custom road, they don't connect within the game )
3. You don't have to worry about SP, the game does it for you.
4. The Brew of Lethe was placed in both #214 and #271. It likely says "(delete)" because there are two of the same item.