Can end user release messages in quarantine applied by mail flow policy?
can end users release spam messages in end user quarantine even if spam notification is off?
They can, by using the SCC: https://docs.microsoft.com/en-us/office365/securitycompliance/find-and-release-quarantined-messages-...
Notifications are just that, notifications. The lack of notification doesn't prevent the user from releasing messages.