Ok. May have an idea which can get this type of file printable. The issue with the file is non-manifold surfaces embedded within walls, underground, etc. However, the land/feature surface of the file is correct.

The idea I've had, and begun testing is to texture the surfaces of the model with a high resolution texture – something with lots of pattern on it and no large plain areas. Then, rotate the model in perspective view and screengrab the images at a high resolution. Finally, run those images through photogrammetry software. This will reconstruct the visible surface and make a waterproof surface, whie avoiding all the non-manifold surfaces embedded in the model.