Based on the description and marketing of the DCI championships discs, I get no impression that the video and audio for the DVDs and BluRays are only from Finals. I would hazard a guess that the video clips are spliced from all available photoage. Using that stradegy allows for the camera crews to focus on different parts of the show each night and gives the editor the most choices for the final cut.
DCI corps are so well practiced and consistant at their shows, that I would also guess that there is inter-song audio mixing going on, not just inter-show, but I can't be sure. I don't think it is false advertising or detertrimental to DCI or the corps featured, since it is impossible for the video to live up to the live viewing. There just aren't enough pixels and fast enough bit streams to support that deep a level of interaction right now.
The high-camera video (not audio) is the only photoage that I can think of that is most likely entirly from one night.