Is there a functionality to make tags optionally global or individual or introduce a new mechanism for emails to be tagged for deletion that will only impact that mailbox but still allow an automated process to run to do this?
Can users define their own Retention Policies?
This is not currently available. Change Request TF00210286 has been raised for this future to be included in a future release of Archive Manager.
There is not an easy workaround for this one currently. If there is a need to accomplish this for a few users, a delete policy could be created for those particular mailboxes based on a tag that those users can apply, if they have the View/Edit tags security action.