1.
Yes, You can build on ANY map that has a flat surface.
2.
If you want to build interiors you will have to get a 3D modeling program or an interior program, I think it's 'Torque Constructor', but i'm not sure. And if you want landscapes just modify the target of your shortcut file.
3.
The Mission editor, just a window that comes up. It shows the X,Y,Z axis and has other editors with it (GUI editor)
4.
There are quite a few people that play Blockland. You will make friends fast, unless your an idiot. All of the people that play have payed for the game, besides Badspot (Maker of the game) and possibly some beta testers.
5.
You have to use your avatar to build. The default controls Badspot has chosen, can be mastered fast. When you master these controls building will be simpler than when using legos. I can make a big house in under 15 minutes.
Buy Blockland end of story.