So, if I understood you right you’re saying that the terrain editing code is in the editor (and that one’s not allowed to link to editor libraries in a built game) and therefore not available runtime? Is there any limitation why it couldn’t be put into the game runtime library, or was this just a design decision at the time to put all the terrain editability into the editor code?
Out of curiosity I did download the sources and peeked into the Landscape files, but understanding what’s happening and where takes some time and feels quite overwhelming - the related code is spread all over the place.