Here's an update on Crash. She's getting back to her old self. She made it onto the bed the last three nights and for the last four days has made it onto the couch. I even picked her up tonight with only her usual protests.
Looking back, I can think of three reasons for her listlessness and weakness. About two days before I noticed symptoms, she tried to jump onto her sink for her drink of water, but she couldn't make it. I just assumed she was clumsy (I've had a lot of cats like that). The next night when I was trying to get her to go to bed, I tried picking her up in spite of her protests and might have pulled a muscle.
But, I'm inclined to think that the real reason for her "problem" was the fact that I had put her on a crash diet. I drastically cut back on her food because she's getting too fat. She was probably weak from hunger, that's probably why when I was testing her, she ate everything I gave her. It looks like she and I are not able to diet and lose weight.
She hasn't been too happy about her diet. Maybe we should jog around the house, or even walk the length of the house about 400 times. That should make a mile. Exercise instead of starvation.