The parts are mostly independent, but you should read the first part before the rest. We don't have the time to send out missing parts by mail, so don't ask. The Cryptography FAQ is posted to the newsgroups sci.crypt, talk.politics.crypto, sci.answers, and news.answers every 21 days.
Nobody knows how to prove mathematically that a product cipher is completely secure.
For example, the cipher must be nonlinear, and it must produce ciphertext which functionally depends on every bit of the plaintext and the key. The nonlinearity of a product cipher reduces to a careful design of these S-boxes. The security of multiple encipherment also depends on the group-theoretic properties of a cipher. If for every K1, K2 there exists a K3 such that eq. In their paper they give strong evidence for the hypothesis that DES is not a group.