a. The mirror locked up can be released by depressing a tiny black button on the front left ( if you are facing the camera ) and then hitting the shutter button.
see attached image
b. The early 70's Pentax lenses sometimes used glass with trace amounts of radioactive elements that decay over time causing this "yellowing" effect. You can undo this by letting the lens sit and get hit by sunlight ( UV ) over a period of time. Otherwise, the yellowing will warm the photos you take. The lens is NOT dangerous but I wouldnt keep it close to your eyes for any length of time...see http://www.photo.net/bboard/q-and-a-...?msg_id=00D80Y
c. read about Pentax 67 lenses here