1. There is no Red Hat Linux 3.5. What exactly are you referring to? Red Hat Linux 9 got released about 4 years ago and was superseded by Fedora Core. There is "Red Hat Enterprise Linux". Are you referring to that? In this case it isn't quite actual, but you might be able compiling the needed tools.
2. See the wiki
3. design artifacts? Rockbox is written from scratch. There are "design artifacts" from the older lines of supported players if you'll call that "artifacts". Look in the code, you'll see.
4. see docs/ folder