Quote - 14 miserable days later, 14 days of constant one-step-forward-two-steps-back, and I have managed to produce only one - hugely flawed - render.

Maybe its time to try a new renderer.  If you have a decent nVidia graphics card then I would recommend the new GPU accelerated Octane Render which is currently being offered at half price during beta testing.  Save out in obj and import into Octane for rendering.  See if that solves your problem.