Knoppix is designed as a live CD and as such has a mix of stable, unstable and experimental sources, not recommended for hard drive install unless you are an accomplished linux user.

If you want a stable system then why not download Debian Etch and use debian proper?
To answer your questions:

1) If I understand your question, then yes, knoppix is debian compatible, you can use all apt-get commands including deporphan

2) No. Knoppix will not upgrade, and even if it did something would break. Better to backup your software and do a fresh install of a newer knoppix

3) If you install knoppix, then chose the debian option, the live install option behaves as the live cd which is why you have to configure
internet access each time