You don't actually need 400 persuasion- sorry if the guide is confusing! Having 400 persuasion just +1 right so there's more breathing room if you didn't choose enough correct answers before. That's same with the other ROs, there's no persuasion requirement but being more persuasive helps. Clarmont's only special because he needs you to complete the rebel chain.
And yeah rebel approval doesn't matter for Clarmont romance. Failling the rebel approval check during the final rebel secret gives you 50 Sayra and Clarmont rivalry, but I don't think the game ever checks for your rivalry with Clarmont if you're already romancing him. However, the Sayra rivalry blocks her from revealing her identity in week 6, so anyone who wants a good revaire ending should try to stay on the rebels' good side.
If your widow betrays Clarmont, then that's not a problem for you ;p