Regarding the 30 seconds, was the camera set to mirror up mode (Mup)? If set to Mup pressing the shutter button raises the mirror (which may sound like the shutter opening, but is only just the mirror raising), but does not take a photo. If the shutter button is not pressed a second time, to actually take a photo, the camera will automatically take a photo and lower the mirror after 30 seconds. Basically Mup can be used as a 30 second timer.
Not sure about why the photos were dark unless (1) a lens cap was on or (2) all the messing with shutter speeds accidently resulted in severe underexposure.