The nodes to the left of the ggx shader are the roughness value (just a float value) and a multiplication node it is plugged into, multiplying it by itself to get roughness behavior that uses values that translate better to the ones you are used to working with in substance designer/painter. They explain it a bit in the mdl intro tutorial that they put on the official allegorithmic youtube channel. For the IOR, I notice you have a node called 'abbe number' plugged in. That may work, but a simple float value with a number around 1.5 will work fine and is what I used. As for the iray to open gl conversion, I'm not really sure what to tell you on that. Any real time (directX/openGL) subsurface scattering approach (currently) is going to use a lot of approximation and work very differently from how it does in a ray traced solution like iray, as you don't have time to compute the ray traces required to get real refraction. In the youtube comments of the existing mdl tutorials, allegorithmic have said they will be doing more tutorials shortly covering stuff like this (SSS etc), so maybe they will address that and be able to help you.
