If I'm in an elevator and the mechanisms fail causing the car to plunge at terrifying speed, if I jump at the very last second before the car crashes into the ground will I be okay?

It will help a bit, depending on how powerful a jumper you are.

If the elevator plunges a distance H, and you are capable of jumping a vertical distance h, then jumping at the last second will reduce your velocity and change the "effective" height of the fall to:

H = (H1/2 - h1/2)2

So if the elevator falls 50 feet, and you can jump two feet in the air, then jumping at the last second will make the fall only as bad as if the elevator had fallen 32 feet.