Greg Hughes

It is simplest to just attach a Material node at Geometry level to the Object Merge and select the correct material from the menu on the right of the Material file slot.

When I use a Layer Mix node to mix materials in the Material Network, the textures no longer show in the viewport, the object just gets grey shading. I have to render to see them. If I just use a basic Principled Shader on its own, it shows in viewport. Seems to be a limitation of the viewport renderer.

Is this just a problem confined to Macs running Houdini, and is there a fix?

Do you mean that the Settings tab-Threshold Range acts as a mask to only allow disturbance in that range, and that the Strength of disturbance is then multiplied by the temperature value in the range so that lower density areas in that range get more of the disturbance affect?