None of the formats you are working with have any georeferencing data in them, so this is not an issue with georeferencing per se. That probably explains why it doesn't matter whether you use the geog image map shader or normal image map shader. Because the positioning is not determined by georeferencing in the first place.
If I am understanding the files in your download correctly, you have a terrain (.ter), along with several image maps, all of which are in the same shape (aspect ratio) and projection, and are intended to be overlayed on top of each other and wrap completely around the planet. My suggestion then would be to actually use Image Map Shaders for *all* of your images and use *spherical projection*. Do not use Geog Image Map Shader as it does not have a Spherical option. Basing your work on BigBen's project is probably not the way to go because his project involved using high resolution *partial globe* images, not intended to wrap around the whole planet. The only way to do that is with Spherical Projection.
So, first you need to get your .ter into an image format so you can load it in an Image Map Shader. Right-click your Heightfield Load in Terragen and choose Save As, then give it a file name with .exr on the end. You can then load the exr into an image map shader, and you'll want to enable displacement and then set the multiplier to *the altitude of the highest point in your terrain* (in meters). Load all your other images into image map shaders all connected to the planet as well, similar to BigBen's setup in how they're all configured, but using Image Map Shaders instead (in fact you can still start with his basic setup, just replace the image loading nodes with standard image map shaders). Set them all to Spherical projection and then, this step is critical, you need to set the position of all of them to be *the same as your Planet object*. Basically just go to the Planet object and copy the Position coordinates and paste them into the Position parameters for each of the Image Map Shaders.
Once you've done that it should all be wrapping properly around your entire planet. And if I understood your files correctly, that was your desired result. If not perhaps you can explain further.
In the future I suggest using the "Export gathered project" feature from the File menu if you want to share project files. This will properly reconfigure your project so that file paths will work on other computers. As it was your TGD just didn't not work, none of the file dependencies could be found without manually relinking them all.
- Oshyan