Ideally quarantine notifications would ONLY get sent to a nominated admin account (could be global admin account).
Disagree. Let user manage his own quarantined email will reduce sysadmin's workload. And only the user can 100% sure which one is spam.
What would also work nicely for us if there was a way of cc'ing the domain or global admin account on each quarantine notification email sent out to a user.
This is possible, technically.
1) Leave Self Service disabled for all users.
2) Modify the notify_quarantined_recipients.html so that the notification email is worded in a way that directs the user to their domain administrator.
You can generate file "tools/notify_quarantined_recipients.html.custom" (which is a copy of notify_quarantined_recipients.html with your own modification), it's template used as email notification, you can hide the link to iRedAdmin-Pro self-service and add whatever you want.