Current questions and answers regarding the map editor.

There, where the texture properties are, are the actual texture properties. That is, how it should be applied. For example, the ghost model has a special property needed to support semi-transparent textures. Perhaps something similar is set here as well. And by "empty" texture, I meant not null (because then the mountain would be covered with a "standard" texture), but rather a completely transparent one.

And how do you create a mask on the mountains so that they cannot be walked on?

There are also the tiles and objects tabs, and there is also a masks tab. There you need to select and apply yellow squares to the map with a brush.