> Maybe you hadn't heard, but there are people being paid to work out how to do all these things,
Yup, I'm one of them. I'm working on the idea of a "Seed Factory" ( http://en.wikibooks.org/wiki/S... ), which is a starter kit of machines which you expand by making parts for more machines. A 3D printer is likely to be part of the starter kit, but you need several others. The engineering R&D questions are what machines should be in the starter kit, what is the optimal growth path, and how do those vary with available raw materials and the desired end outputs.
The point here is sending a whole industrial plant into orbit to process an asteroid is much too massive. You want to bootstrap up from a minimal starter kit, and build the rest out of the asteroid itself, as much as possible. You won't reach 100%, some stuff will still need to come from Earth, but saving 85-98% of the launch weight (what we think is a reasonable goal) is still a huge advantage.