You were right - they weren't connected. I got the right result:

// Result: Connected floor.message to mia_material_x4SG.miMaterialShader.

Unfortunately, my render still isn't any good. It's my first one, so I'm really happy with it except for the texturing issue - as you can see, it's affecting all of my image textures. They're all high res, and I was careful to map them properly .. Any more help would be greatly appreciated