If I am creating a master sendable Data Extension to hold all of our email subscribers, and our account uses a person's email address as their Subscriber Key, does my data extension need to contain both Email Address and Subscriber Key columns? If so, does Email Address need to be the "Email Address" data type and the Subscriber Key the "text" data type? Finally, which column needs to be the "primary key" to relate to my All Subscribers list in order for unsubscribes to be processed correctly? Thank you.
It depends on what data is contained in your SubscriberKey. The SubscriberKey is how SFMC keeps track of a Subscriber's status. In most cases it's the email address, but if you have any kind of integration (e.g. Marketing Cloud Connect), the SubscriberKeys are typically not email addresses.
To configure a sendable DE, you'll have to define the relationship of the DE data to All Subscribers so the status can be maintained. You don't need both email address and SubscriberKey if your SubscriberKey is email addresses. If they're different, I recommend having both.
Primary Key is to define uniqueness of a row, so no two rows can have the Email Address + SubscriberKey combination, then those two fields should be set as Primary Key.
If you want to use a Query Activity to update a Data Extension, you'll need to define the Primary Key fields.