I wouldn't say that there is no serious literature on fasting as Shelton plus Bragg and many others wrote volumes on the subject and from their own experiences with supervision. Bear in mind that Shelton supervised over 30,000 fasts in all manner of health and disease and unparalleled since his time.
Does anyone have to fast themselves to observe the effects it has on the human organism? Not essential but useful I would say.
" Shelton believed that fasting was for sick people, and he did not include himself among the sick. His preferred method of maintaining health was his diet".
Fasting when well is superfluous, and that is why Shelton used the fast in diseased states, just as animals in the wild will fast when sick or injured or have to go without food involuntarily through scarcity. Rarely if ever do animals fast when well, unless at times of hibernation through the winter months.
Bed rest conserves energy to the max' extent possible, to devote this energy to cleansing and healing exclusively, rather than waste any on activity or emotional involvement; this is also apparent in the wild where fasting animals will hardly move except to obtain water for survival.
So most of Sheltons practices were based on Nature and what happens in Nature rather than anything else.
Enemas were abandoned for the most part within Hygiene because they were found to be enervating in the sick, and therefore counterproductive in recuperating nerve-energy or the life-force.
Rather cynical to conclude that Shelton encouraged bed rest in his patients for his own convenience: do you have any proof of this? Purely conjectural.
Shelton did encourage walking for some within the limits of their energy and their health-state, and he actually did have the grounds where this could take place.
Gymnasiums, saunas (heat-enervating and therefore dangerous on a fast), colonics (enervating) are a waste of time as autolysis will use impacted feces as fuel during a fast, so again entirely superfluous. They do not facilitate a fast in any way, but quite the opposite.
Fasting clinics will use these as a justification for the expense of staying there.
Shelton did originally use the enema and colonics and other what he referred to as "forcing measures", but eventually abandoned them through experience because of little if any benefit, and sometimes their distinct harm; he referred to enemas and colonics as a "pernicious evil".
Inactivity will produce muscle wastage while eating, but while fasting, muscle is greatly preserved through something known as "protein-sparing", so you are incorrect on that score.
Bragg and others did encourage the use of distilled water, as we do here, and with very good reason, so this is not a blunder or a mistake either.
If you would care to cite the studies or works in PubMed I would be obliged, but most of them were done incorrectly so have little value.