When I find a quote I'll post it but:
1. I've read that when the CA was being written Roboute was compiling all the tactical and strategic knowledge available. He wrote the words themselves, but that's not to say some of the other Primarch's wisdom did not help him along.
2. The Apocrypha of Skaros is the oldest known copy of the CA. Earlier copies are lost. As such, it may differ from the original, but it most likely differs the least.