you wrote "I base my views on muscle wastage which occurs more quickly when exercising during the fast on the research work of Dr Joel Fuhrman MD and a few others."
Research work? What research work? Citations please.
This contradicts what experienced fasters/bodybuilders say. The speculation about muscles and amino acids is just that, mere speculation. It sounds reasonable, but it does not mean it's right.
If you read that Springer book on contemporary research on fasting, you would learn that, for example, the migrating geese, who mostly fast during their migration, do not loose their wing and leg muscles -- not at all, while their GI tract atrophies and they lose some non-essential muscle mass. The old adage, use it or loose it, applies also to fasting.
As for "protein sparing" this merely relates to ketosis -- which is always a feature on a fast lasting more than 2-3 days. It reflects the fact that when the body runs on fatty acids and ketones, rather than on glucose, the demand for protein to make that glucose goes down. This is what 'protein sparing' means.
In addition, exercise, besides having many other positive effects, upregulates the Cori cycle where lactic acid is recycled to glucose, diminishing the demand for gluconeogenesis and so also contributing to the sparing of protein.
And why did you decide that the metabolic rate is lowered during the fast? Metabolism is certainly different. But how do you measure the metabolic rate? It's not just calories 'consumed' from endogenous fat and protein. The efficiency of metabolism goes up due to the upregulation of mitochondrial function. It so happens that exercise also upreglates mitochondrial function, that's why many fasters crave movement in the beginning of the fast.
It takes 2-3 weeks to adapt to the ketosis of fasting, and during this period the daily weight loss dwindles down to mere 150-300g from a kilo+ during the first days. Yes, the body conserves energy during a fast -- it would be stupid if it wasted it. That honing of the metabolic processes is what gets rid of everything superfluous, unnecessary and even harmful. The weak, old and diseased cells perish. That's how fasting cures and rejuvenates.
And I'm glad you brought up caloric restriction. According to the research, it is the total number of calories consumed in a given period that qualifies a diet as CR (or not). Consider that with regular fasting one can eat normally on the fed days instead of continuously experiencing hunger as is often the case on CR.
Also, the latest CR experiment with monkeys showed no life extension due to CR, even though it did show extension of many health parameters. I believe the difference in the results compared to mice was due to the fact that those monkeys did not fast proper, while CR'd rodents do.
The mouse metabolism is 7-8 times faster than human. So, a mouse fasting for 24h is metabolically equivalent to a human fasting for a week. Those CR mice are usually fed once a day, which they gobble up in a couple of hours and then they fast for the rest of the day, actually going into ketosis and loosing ~10% of their body weight in 24h period.
Also, most people prefer to eat their full on the fed days and then fast periodically. In this scheme the amount of calories consumed per year is probably even less than for someone on a 20% CR, depends on how often a person fasts.
In the end, how we choose to limit our calories, day by day or in chunks, is a matter of personal preference. I'd say that periodic fasting is more natural (we agree that habitual overindulgence is what drives most modern diseases).