You can save terrain data in the js file I believe. But I don't think you can open the pmp directly. Check jebel barca for an example on how to use a height array.
I do not know much about the GUI save from the very basics. I'm hoping @Nescio
@nani @badosu
and @ffffffff
can give you some pointers. Maybe help us improve the documentation so it's easier to find information.
Well you need to delete the materials. And if they somehow use different textures you have to either
Merge all the textures into one and tweak the uvs.
Select your model go to edit mode press P and select split by material then export every different pieces one by one.
We you could create a special actor with the field mesh and a custom template with obstruction. This way you wi have a custom obstruction object. You can set it to be only visible in atlas See the trigger markers for example.