Marketo keeps a Customer Global Unsubscribe list. That when you deleted a previously unsubscribe record and then by intentionally or by accident import that record again, they will be unsubscribe as default on import.
Once the record is gone there is no history or field to listen to in a smart campaign to change their status when you reimport again.
Does it mean unsubscribed records can be deleted from the lead database as Marketo will mark the same email address as unsubscribed when it is imported again?
For invalid email addresses, they have to be stored in the lead database to keep them flagged as invalid?