1. You should be able to find it in the manual directory of the Girder install.
2. C API is still in use, yes.
3. Speed. If you have a lot of data to process in small chunks C is preferred. Otherwise the transport API is quite good at handling large amounts of data if the chunks are large enough (e.g. not calling into lua a lot)
4. Get Visual Studio, one of the free editions will do I believe.
5. Just the default of the forum, simply ask it to show you more threads at the bottom. (Last year or so ).

Most people these days develop plugins using the transport API hence this has gotten quiet.