I'm not sure if it satisfies all your needs, but Scheme 9 from Empty Space is a full R4RS Scheme implementation written from scratch in pure C (and Scheme). It has a heavily commented source code and a book, which is supposed to be quite good.

Here are some key points from the website which the book addresses:

- How does tail call elimination work?

- How is macro expansion implemented?

- How is arbitrary precision arithmetics implemented?

Enjoy! http://t3x.org/s9book/index.html