How to give credit? Just put a text file that says modified from such and such by so and so?
Include a text file (generally named "Authors" or similar) with your mod crediting all creators, generally including the type of license with it.
The building itself is invisible.
Most likely your model is in quads, not triangles. In edit mode, select everything (
a) and press
ctrl + T to convert all faces to triangles. If that doesn't work, upload the model so we can take a closer look.