The flashes on the stage have to be set to remote mode, of course. If you are using the Nikon Creative Lighting system that is.
the .DSC file is just there to tell the OS or a program in general that your SD card contains data from a digital stills camera (DSC). To find your video you will have to navigate into a folder that is quite likely named DCIM. The extension of Nikon's video files should be .MOV.
Manual focusing is quite simple. Turning your lens's focus ring will override the autofocus or if you switch your lens from AF to MF you will be able to focus manually throughout the video.
Mainly the 600mm/4.0G VR, but also the 400mm/2.8G VR.
See video description for further details.
AF-S 70-200/2.8G VR
AF-S 24-120/3.5-5.6G VR
AF-S 60/2.8G Micro
Don't worry, the D7000 should come with the "BM-11 LCD Cover". By the way, did you know the D7000 is quite rugged? Before worrying about the camera's survival a photographer might actually have to worry about his/her own well-being. Just saying this because there is no need to treat the camera like a baby.