Yep the documentation is lacking in regards to Groups, so maybe you worked it out.
Regarding gamma, make sure your diffuse has 2.2 gamma and the rest should be 1.0 (when you load the bitmap you can override the gamma)
In regards to opacity just use the opacity slot/texture, dont use an inbult one in the diffuse. If I understand you correctly.
I dont use the plugin because I have built my own PBR/Kodama pipeline. I know 100% my models are PBR and real-time ready in 3dsmax,c4d, blender, fbx, obj,gltf,glb. I dont care for their Kodama badge or certification, it means nothing to me. What matters to me is that my clients get what they paid for and it works every time.
No you cant use physical/standard materials for corona/vray. They must be converted. I have also built plenty of scripts for artists who want to convert their Physical materials back to Corona, or Vray to GLB for real-time exports, you can use them they are absolutely free for the community and built for people like us who sell content (check the readme since it also has youtube videos showing you step by step how to use the scripts):
https://github.com/jmdvella/3dsmax-scripts
https://www.youtube.com/jmdvella
I also export my models to other formats like you intend to or do already, and thats why I build these scripts.