Well, naturally, as soon as I post this, I stumbled on part the problem. Even thought I overwrote new materials, the textures which I thought were gone were still there.
I am still uncertain why they are not eliminated when overwriting though. In fact, I just tried switching back and forth between realistic materials and all white. The textures come back as they should when applying the realistic materials but they stay when applying the white.
When I click the overwrite button, the white materials all show up in the left hand pane with no textures indicated but when I hit apply, then modify, the texture names pop back up.
Anyone know what's up? Am I doing something wrong?