1. Yes, it does, but only the cparse.d file is for C. The rest of the "C" compiler hijacks the D semantic routines, code generator, and optimizer. There's also some custom work to turn #define's into simple declarations. If you just want to parse C, yes, you can just use cparse.d.

It's important to note that C cannot be fully parsed without keeping a symbol table, and cparse.d handles that, too.

2. C extensions will always be a problem. ImportC supports the most widely used ones. (Most C extensions are rarely used and can be ignored.)