1. 90% of the materials I use are picked from the common library I use for most of the projects. the other 10% are usually based on those but tweaked to clients’ specification…so yes, in maxwell (my tool for years now) you have the MXM extension which stand for maxwell material (also with external editor that I never use). almost never I’ll try to create a material from scratch unless no other option to play with something from the library (that includes all typical materials with easy to change attributes).
2. I never use referenced materials files as most of the clients ask for “change this change that” and since material files are usually quite small I don’t need to save space and wnat to have the option to change materials in the scene file itself.