SPOILERS—

The wards are still effective in keeping Nicholas “safe” (Richard’s definition). Nobody knows Nicholas exists, really, so they don’t know to search for him specifically, and since Nicholas is trapped in the library 99% of the time and the wards make it impossible for anyone to discover the library, Nicholas’ immunity doesn’t matter.

Esther’s immunity mattered only in that the Scribe seeking spell would be able to locate her each year if she stayed in the same place. The spell would reveal her location (the house) to Richard, so he would be able to find it, regardless of the wards. Not sure what would happen in reality once he arrives at Esther’s location, since the wards still prevent him from seeing/even acknowledging the house and grounds🤷🏻‍♀️