Hi! Some notes about your questions related to the Houdini plugin:
1) You can use the LiveDB material library ... they need some cleanup once loaded in Houdini, but can be a good starting point.
2) The 'medium' pin in the output VOP is used for the volumetrics shading, to link the volume medium node.
3) This is a limitation of Octane, it is not available even in the Standalone.
4) Yes, usually the Octane users are mixing two or more material nodes using the materials mixer to build complex materials.
5) The lights in Octane are also objects (with an emissive material) so I don't think so, but perhaps the render layers feature could help, I am not sure.
6) You have a feature in the render target imager node 'Hot Pixel Visibility' that perhaps could help with this problem.
7) Linking a texture to the diffuse material 'transmission' pin, usually mixed with a glossy material to have specular reflections.
8) Currently you can use packed primitives or polygon soups if you convert them to polygons for the render SOP. The support of polygon soups can be possible in the future, but the packed primitives express a procedure to generate geometry at render time in Mantra. They can include geometry, external references, HDAs, scripts, mantra procedural objects, etc, so they encompasses lots of features, some of them not easy or even impossible to support by other render engines due to the procedural nature and the Mantra only related features of the primitive. Of course you can "unpack" the packed primitive to render it in Octane, but to render heavy geometry or large number of copies/instances, I want to add in the near future the support of Octane proxy objects, loaded at render time.
9) Because a TitanX must be more expensive than a 980ti to not be a 980ti?