Those are two separate questions.
1) If you delete them out of Marketo (and not Salesforce) then sync them back over from Salesforce, the unsubscribe will remain. Otherwise, Marketo will no longer 'know' if they were unsubscribed because you deleted them (and their lead history).
2) You can upload a list and not update fields by managing your preferences in the admin section. You can identify which lead sources you want to update leads and which lead sources you don't.
as a complement to Kristen's points :
- Read this doc : Durable Unsubscribe
- Keep in mind that you cannot overwrite the "unsubscribe" field though import (while you can set it for leads that are created though import).
Hope this helps,