Hello @Anonymous,
The current AO is simply an opacity map on a black standard material with no BRDF, and the original post was using Scanline which doesn't have BRDF. I'm working with what you gave me and I think the goalposts are moving here. I can try to address your new concerns anyways though.
The Arch & Design materials were never meant to be layered, and the composite material is old, not built for MR, and not fully compatible in the way you are attempting to use it. It still exists to support old scenes but I wouldn't use it with physically based materials, as I've noted. So you have some options.
1) Blend materials within Blend materials. This is what people generally do here.
2) Use composite maps for Spec, Diffuse, Gloss, etc. but having differing IOR's would be difficult.
3) Use a PBR texturing approach on a single A&D material, but having differing IOR's would be difficult.
4) Experiment with the caustics box workaround you discovered, but again... it's not going to be correct with the composite material and especially not with 4 stacked A&D materials and will take a very long time.
5) You might be able to look into the Mila or MDL shaders for 2014 but they need to be downloaded and installed, and that process is not one I am familiar with as they are provided by NVIDIA and weren't integrated in 2014. NVIDIA would have more information about that most likely. The only link I could find was for 2016. I really can't help too much with this if you go this route.
Best Regards,