If the administrator deletes the entire database, are the opt-ins deleted as well? Will all the contacts would have to opt-in again?

If the administrator deletes the entire database all the opt-in are deleted as well. Please remember that if you upload a new contact list, the previously existing database will be deleted. In order to avoid deleting contacts you wish to keep, you will need to upload the entire database; current contacts plus new contacts in one spreadsheet. So, you would use the file you originally uploaded, add contacts to it, and upload the entire file. 

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