I have a question about the Codex Astartes and the Second Founding.

Did Guilliman force his brothers to adopt the Codex? And did he write it alone?

The fact that the Chapters are supposed to be independent makes it a bit hard for me to follow. You have a guide on how to run a chapter (written by a Primarch no less), but no practical means of enforcement. It's just up to each Chapter how seriously they want to take it. What was the point then, other than to annoy Dorn?