hummer17 - Unfortunately the Nav system will not play DVD movies. Yes, the Nav DVD must be inserted in order to use the Nav. I do not believe the system can load your route into memory (allowing you to remove the Nav DVD), but I will check the manual later.
Dan - I don't think you can lock the tire mount. Here's a close-up shot:
You basically pull out the pin and lift up on that top black piece - no mention of a locking mechanism in the manual.