Considering the poor state of the Landscape Studio, if I were in your shoes Norb, I wouldn't spend a single minute of my time trying to make it properly work.
Just move on to Blender or any other stable and well-tested tool to arrange the scene graph transformations, camera stuff, etc.
We can create a dummy terrain node attaching all the required configuration data, heightmap, splatmap textures (in Blender you can also do texture painting), etc.
You can have texture and mesh data embedded or pointing to external references indifferently.
Then we should import from gltf into the existing PR engine structures. The data must resemble the old one, so it shouldn't be too tricky to achieve this.
TinyglTF is a good header-only glTF loader lib and could probably help here.
Finally we'd need to make some python-like tools (or just reuse the PR LoadLevel function) to convert en-masse the current maps/lvl chunks into the new format, saving the huge amount of hours, passion and care map makers put in to create and assemble them.
Sorry for not being of much help apart from popping up with these high-flown plans...
But they were really sticking in my mind since years and what happened here was such a surprise!

Any reborn Ney around offering to bring me back to the zoo in an iron cage?

Cheers.