For now I'm rounding the problem with this:

1. First I ad Diffuse material
meditmaterials[1] = Diffuse_material ()

2. I add all the textures manually from material editor

3. If I need new material with diffuse_tex I make copy of meditmaterials[1] and I can give new bitmap parameter to NewMaterial ...

Yes I did. Could you tell me how your material LiveDB makes the trick when you press "place to active slot of material editor"?

I got same issue! I can only switch texture map if I first manually add texture map from material editor. Before that that .diffuse_tex is undefined and I have no idea how to change that with maxscript.