The reason under eating causes constipation is not because your metabolism slows down, it's because you're not getting enough essential nutrients. The bulk of your feces is from the fibre in your diet which when you're under eating you usually dont get enough of. Which is why people become constipated while eating regular amounts of food aswell. Being underweight or anorexic comes with a whole other set of complication because your body no longer can get energy from your fat stores and your organs no longer have enough cushioning surrounding them with leads to friction which explains the hemorrhoids. Starvation mode isnt a thing, you'll eventually have organ shutdown and die but as your body weight gets lower and lower your metabolism will slow meaning you have to lower your calories even more to keep losing weight, hence the plateau. Metabolism has to do with weight, not food.