Very good points. I investigated this further and eventually I realized I made a few newbie moves:

1. Poser material layers work primarily when each layer has some level of transparency or mask to allow the material layer beneath it to be seen in the render. So the initial setup I shared is completely wrong. The base layer contained an emission material that was completely hidden by the layer above it. A really basic mistake on my part but also now I have a much better understanding of how material layers work and how powerful they can be.

2. True enough, the emission node should not be used at all (for a real world material). I've removed it. To do a "real world" material requires high sample amounts in the superfly render settings (a relative pain depending on the power of the nvidia card). I've read some posts of people using some CRAZY numbers for sampling to get acceptable results. Again, crazy is relative. But here are my results using 50 samples (single light source, with medium-grey background and ground set to visible):

In this render, same as above except with black background and ground is hidden:

I also plugged an ice bitmap into the GlassBSDF's roughness which obscurred a lot of noise but at the cost of transparency (no surprise there):

So basically, yes "realistic" ice cube like texture is do-able but at the cost of high sampling in the render settings. One cheat is to plug the Emission node into a Poser Surface volume and adjust the brightness/color of the emission to your preference: