I agree with your advice not to do a long-term fast until you are fat adapted. If someone goes on an extended fast without being fat adapted, they will get fat-adapted doing it. But the process of getting there will suck. Getting fat-adapted through keto before fasting of any kind is going to make it a whole lot easier (lowered appetite, fewer or no cravings), and you may see greater benefits since you don’t have to burn through a whole lot of glucose from carbs in your system every time you start a fast.
IDM says they occasionally put very ill patients on extended fasts immediately (and note that these patients are being closely supervised by the clinic). But generally they recommend getting into fasting more gradually, so that people build their fasting “muscle.” They also recommend a keto-type diet, but say that it isn’t required. Dr. Fung says one of the reasons he started having his patients fast instead of putting them on a very low carb diet was that it was too complicated for many of them. Their studies and others haven’t been done on fat-adapted patients and show that fasting under these conditions doesn’t harm the metabolism, unlike calorie restricted diets.