He's making changes to the secondary. Hence the LSNs won't be in sync after that. I don't read that as asking how to switch log shipping around, just how to test the secondary without breaking the log shipping to it (and that can't be done)

Restoring a log WITH RECOVERY will bring the DB online all right, nothing wrong there. If changes are made to the secondary, it can't be just backed up WITH NORECOVERY afterwards and have the log backups from the primary continue to be restored.