I don’t agree that overeating at a meal could be accurately categorized as impolite. I also don’t agree that grabbing a second helping - of anything, but particularly salad - at an all you can eat dining hall would be considered overeating by most Americans.
Fasting is probably not likely to exacerbate depression. On the contrary there have been quite a few journal articles published about the benefits of intermittent fasting, which is what OP describes.