1. Good idea.
2. Destroying is much easier, assuming there is no enemy military to protect it, but that's not the case.
3. Agreed, we can always make the map bigger but I honestly think it'll just make all the calculations more complicated and cause more errors.
4. Yes, the problem is just to keep things as simple as possible. If I make the benefit of tech a power function below 1 like it is now, people will just start out taking some tech and leave it as it is until they've completely developed their nation. If I make it a linear function, people will either focus almost only on tech because it gives bigger benefits than infrastructure, or again wait until they've developed all of their nation. Making things require tech in mid-game is a good way to get it into the mid-game, but the problem is that it makes the rules complicated, people might forget it and I might forget it as well when I check the maps.