Let them. Making minor mistakes is an essential part of learning. Your role is to prevent small mistakes from becoming big problems, not to prevent them from happening at all. Eg - if they go the wrong way for 100m, but it isn't likely to be fatal, quietly let them. If they look about to lead the entire group down a climb that's incredibly hard to get back up, and is the wrong way - intervene.

Try to offer advice privately at decision points (e.g., junctions) or before entering a challenging section. If it's a direct safety concern, speak up immediately.

Not quite. A seconder has formal responsibilities like helping with the callout. A backseat leader has all the underlying responsibility of a leader, but should do as little as possible, perhaps even less than a seconder.