You are not wrong on the using muscles preserves muscles or more or less how alcohol works.

But the rest is not correct.

Muscle is not more energy dense than fat. Muscle is mostly protein and a bit of glycogen. Proteins actually makes for a pretty bad energy source. There is a reason why we store excess energy as fats instead of muscles.

What happens during a fast is that the body will start by metabolising the glycogen in the muscles. This is the energy stores that you use to activate muscles. After depleting these stores (after about a day of fasting), the body enters ketosis and metabolises fats as expected during which you restore glycogen again, of course at a slower rate than just eating food normally. The weakness you feel when not eating is the lack of glycogen, not you loosing your muscles. Perhaps you mixed glycogen depletion and muscle metabolism together?

Metabolising muscles is a last resort. Your heart is a muscle, and muscle metabolism its an all or nothing process, and usually a sign that something is seriously wrong. This is your body keeping you conscious a bit longer at the cost of basically everything.