(None of these questions really relate to WebM support, which the thread is about. This should really be in a separate thread.)
The Windows Media Player Preview Handler doesn't autoplay.
But Windows Media Player (WMP) also has an ActiveX control which does autoplay. You can use that instead by turning off the WMP Preview Handler, and making sure the extensions you're interested in are included in the Generic ActiveX (32-bit) line near the bottom of the dialog (the same one where you enable/disable the WMP Preview Handler).
You may also need to fix registry settings for that to work, if WMP isn't assigned as the ActiveX handler for those file types. That can usually be done by (temporarily) setting Windows Media Player as the default player for the types involved (see HOW TO: Enable/fix playback of various media formats in Opus).
You can't, it's always black (or whatever the software playing the video wants it to be, but that's always black in my experience).