both of the claims "1-2 pounds of watermelon/day kills my desire for processed desserts" and "This leads to weight loss, although maybe that also requires potatoes?" seem to me like they can be explained through the lens of volume eating. I am curious what you think of this theory of mine since you clearly have thought about it for much longer. In case you aren't familiar with the concept of volume eating, there was a  done where they gave people 240 calories of a food to people and then measured how hungry they were over the next 2 hours and they found people who ate boiled potatoes were less hungry then people who ate 240 calories worth of anything else they tested (unfortunately boiled potatoes was also the only vegetable they tested, so who knows how it compares to other vegetables), they also found that how filling something is positively correlates with the protein, fibre, and water content of the food and negatively correlates with the fat content. watermelon has a high water content. So basically my theory is that you started eating two foods which are great for making you feel more satiated and then you didn't crave processed desserts or other calorie dense foods anymore because you were already satiated and so you lost weight, your theories by contrast seems more complicated to me. Obviously there is a lot I don't know about your situation (what your diet looked like before the watermelon/potatoes, your strange medical condition, etc.) but I do think it explains that part nicely, I would love to hear any evidence counter to this theory.
Epistemic status: just speculation loosely based on my understanding of how my brain works and assuming other peoples brains work similarly.
I think the problem might be that there are too many mysterious diseases. As an analogy: I have a desire to learn a second language, but often when I start trying to learn a language I will be distracted by other languages I could learn, I'll study one language for a few months and then abandon it for a different language, and then abandon that language for another, until I look back and barely remember any words from any of them, and I'll think if only I had some reason to commit to a specific language, like if I had a partner who spoke a second language, then I would for sure be able to focus my effort to learning that specific one.
In the same way I've researched medical topics before, but there is a lot of medicine to learn and I have no reason to focus my effort on one specific mysterious disease. When me or my family or friends get sick I will sometimes spend some time learning about the disease or the medicine that they got, but quickly move on to other things that interest me of which there are many. If me or someone very close to me got sick with a chronic mysterious disease that would probably result in me getting fascinated by it to a similar degree as you are now, but currently I am not in that situation. other than someone I care about getting sick I am struggling to think of another reason I would force myself to focus on one specific issue, perhaps if the government or my job assigned me a specific illness but I am not a doctor so I don't see why they would do that.
I can only remember times I've heard "Better safe than sorry" used to encourage taking an action that had a high chance of being a small waste of time or effort, with a small chance of minimizing the consequences of an unlikely event that would make it worth it, such as taking an umbrella in case it rains or seeing a doctor in case there is something wrong. I can similarly only remember "Nothing ventured, nothing gained" used as a way to encourage taking actions specifically usually also ones that have a low chance of a high reward, and a high chance of a small loss. These do not seem like opposites to me.