Yes, this service is also available for tickets for 2 days or more. Plots are purchased during the ticket purchase process (if you already have your ticket you can add them at any time by logging into the Buyer's Portal with your e-mail and locator). ...
No, it is not possible: the days of Festival and Camping must be the same, so for 6 days of festival you should buy the ticket "Festival 6 days" and "6 days of Camping".