As for your questions: the emails seem to work just fine. You have to make sure that your IP is not blacklisted. YNH will show your that in the Diagnosis.
Security: for us we use 2FA for everything important and SSH keys. For the users is up to them to keep their stuff secure.
Backups: we have a borgbase.com account and setup Borg with YNH to backup there, daily. Very easy to setup via YNH. I make sure to get an email after the backups are done to make sure they work well.
My #ISP does not offer to change r-DNS records
By the "users" you mean the YNH users? We only have a handful and none is admin but me. Our users for our services are not connected to YNH. So not using LDAP.