The instructions are on the phone itself under Settings--> About Phone--> System Tutorial.
Wifi is simply an alternate connection and provides for faster upload and download speeds compared to 3G. Very important when streaming music or movies.
No, you have to finish the movie before sending it. Limit the video to 30 seconds or the file size will be too big.
As for the holster, it sounds like you have one with a magnetic closure. The magnet makes your phone think it's in the multimedia dock and will turn on every time. You need a holster with a velcro closure.. that will fix it.
Hope that helps!