Hi there ABBAS….
The links seem to be broken … at least when I tried them so I will answer part 1.
Unable to select “unused materials”. To change materials or textures all materials/textures you want to use or switch between need to be loaded into the scene. One option is to take the materials that are not selectable or visible in puzzles is to created 3d planes Blender, Max or whatever you are using. Map the material you want to use to the plane (you don’t care about UVs) for this. Then scale the object so that is small and hidden in another object in your scene or hide it off camera. Export the GLTF and then you will be able to select the new material via puzzles….The trick is the materail you want to use has to be in the scene.