Reply To: Are faces that cover other faces BAD?

I am pretty sure that no Renderer has a problem with non-manifold geometry, after all, a Plane is non-manifold.

But you can give the floor some thickness, if you want to:

I don’t know what is causing that shading. Maybe try changing the Viewport Lighting to see if it’s persistent (try the different options, Shadow and Cavity are also fun to play with):

It could be a topology problem; with the whole Mesh selected, try M > Merge by Distance, followed by SHIFT+N.