Blender = Mix (under Mathematical. Alpha is the equivalent of Blending, Base the equivalent of Input 1 and Layer the Equivalent of Input 2.

FBM I'm not sure if there is a direct equivalent, but the enries under textures would be the most likely place to look.

The DS Default Material is optional - it's a single brick that encapsulates the DAZ Default shader, so the settings more-or-less match that. You can however dispense with it and handle your own integration of diffuse, specular, translucency etc. to give final colour and opacity values for the pixel.

Yes, displacement has its own root brick, though bricks can if necessary send output to multiple roots, so you don't need to double-up common elements.

Remember that all of this applies to 3Delight, not Iray.