I find it easier to use two separate forms - one for preferences, and one for the unsubscribe. On the landing page, the form with the options would be at the top, and the "unsubcribe from all" form would be below. This will guide the eye towards the preferences, while the "unsubscribe from all" is less prominent.
One major advantage is that you don't have to worry about conflicts between selected subscription options and the Unsubscribe field. It is clear that when they click the "Unsubscribe from all Emails" button, you can run a campaign that deselects all subscription options. On the contrary, you know that if they select a subscription option and submit, you need a campaign that ensures the Unsubscribe is "false".
This requires no scripting or other advanced techniques - only custom fields for the subscription options and a few campaigns. You can usually build a prototype in an afternoon, then test and refine from there.