Anyone who knows what OSS is should also know the differences between OSS and free, so (c) isn't too much of a concern - generally speaking, for the most part, etc.
(b) is definitely something to consider, but there are always situations where someone simply does not have the time and/or ability to do their own theme. Case in point - I can't draw to save my life, but I have good eye for layout and colour, so I'm creating a theme. It's taking forever because I can't draw, but I know exactly what I want to see - just takes me a while to get there. If I had $50 or $100 to pay someone to develop the layout I want so it looks like it's meant to look the way it looks, I'd be happy :)
(a) yes and no. There is currently no in-depth FAQ to my knowledge, and I do see *many* of the same questions being asked over and over again. If someone with a couple hours a day to spend on it for a week or so would start one in the Codex, I suspect it would quickly be linked to from the Support page here as a "first-stop" resource for people with problems or questions. At the same time, with regards to Themes, there is *plenty* of documentation on how to build them and tons of examples of what you can do with them. Education is always the first choice of the intelligentia.