Up front: I am biased against extreme diets like water-only fasts. I can see a use case in carefully medically supervised settings, such as for a cancer treatment, and I know that some religious practitioners use them. I've never tried them and have never been morbidly obese.
The only truly releveant paper I found that looked relevant was a case study of a woman whose 40-day water fast caused thiamine deficiency, which led to her developing a severe neurological disorder called Wernicke's encephalopathy (source).
The academic literature on prolonged water-only fasts is extremely limited, particularly for weight-loss purposes in humans. Prolonged water-only fasts have a few more studies in non-human organisms and are sometimes used as anti-cancer treatments.
I would be really careful about basing decisions on reddit anecdotes. Questions I would be wondering about:
The metis in the dieting world appears to be that crash diets tend to be yo-yo diets, leading either to failure of the diet attempt or to rapid weight regain after the diet ends. My picture of successful dieting tends to look more like what I'm doing:
Please don't take this as medical advice - it's just a description of what I'm doing. I'd highly recommend exploring a range of dieting options and considering each on their merits before selecting one, and ideally interfacing with a medical professional to consider your specific situation.
Non professional opinions of a 60kg guy who was full-time obsessed with learning nutrition/fasting for 2 months.
Literature on long term water fasts is severely lacking, most likely because it's not profitable to promote anti-consumption in consumption society. Reading medical studies directly is a great way to understand fasting and nutrition. With better understanding of nutrition it's much easier to see junk food for what it is. And in consequence it loses its appeal.
Here I cite reddit posts, not literature, because /r/fasting has a lot of good anecdotal data, and many weight loss studies are limited in scope.
The answers to any of these questions will likely depend on your starting weight.
On Question 2: In theory this is just a function of your BMR (basal metabolic rate) and TDEE (total daily energy expenditure). For example, if you are large enough to have a TDEE of 3000kcal, then you will lose 1lb of body mass per day (how much is muscle vs fat unclear).
In practice this is a bit of an overestimate. For anecdotal success stories you could go to /r/fasting. On Top All I see:
Searching for "14 day" I see: (keep in mind, about 10+lbs of this is water weight)
Common wisdom on this subreddit is you get 0.5lbs/day of "real fat loss" during an extended fast.
For point one , yes. We have evidence that your body has a steady state homeostatic "weight" that it will attempt to return you to. Which is why on the whole all fad diets are equivalent and none are reccomended.
"Non metabolic" is sort of a vague statement but top of my head besides "organs" i'd imagine the possible gut flora problems could be huge (or it might be great because presumably you have flora right now encouraging excess fat etc)