Overtightned the rear derailleur cable clamp. It is now blocked

I would back off the bolt in a manner similar to your second image. With a small screw driver pry the clamp away from the derailleur toward the head of the bolt. When the clamp is free, you will notice a small "nub" protruding from the derailleur. The purpose of this nub is to prevent the clamp from rotating when it is tightened. My guess is your clamp is on the "nub". This is preventing the clamp from seating against the derailleur and holding the cable in position.