1.d. Why the resurgence in steel over aluminium or carbon fibre? Do you think this is as much down to the process as the evolution of the material?

Process and accessibility. I may get flamed for this, so I'm going to phrase it carefully. Bicycle specific components for steel frames are more plentiful (thanks to many of the builders on this forum), with greater variety, to folks first starting out than aluminum or carbon fiber. Start-up costs for tools and equipment to make brazed frames is lower than for carbon and aluminum (one can make a "rideable" steel frame with only a torch, vise, straight edge/square, files, and a hacksaw) The learning curve needed to be able to stick together a set of tubes and lugs well enough to avoid catastrophic failure is easier than to produce the same lack of catastrophic failure in carbon, aluminum, or titanium. ( I'm gonna catch it for that one, but note I did not say "build a frame") Steel is strong, more forgiving than aluminum or carbon, can be cold set, does not require heat treating after joining, and is far less likely to fail catastrophically. All reasons that have established it as the go-to material of choice for all but a small fraction of the bikes produced in the world.

Full disclosure about who I am- I've only built perhaps a dozen frames, all lugged steel with the exception of my current ride which I built 5 years ago from carbon fiber. I work with a wide variety of materials at my job; I can tig weld steel and aluminum ( will never have the skills of those whose work you see in F.N.L), and done a fair amount of work with composites.