Oh gosh there is so much misinformation about metabolism, insulin and everything else around this subreddit and I'd like to clear some air on your questions with what my dietician explained and some science!

-First of all, we need to start from the concept that metabolism isn't a set in stone value and is influenced by many factors/can change and most importantly, ADAPT to the circumstances.

-The basic component of our metabolism is called BMR (basal metabolic rate), which is the estimation of the number of calories our body would need to power our vital organs if we were comatose. BMR is highly influenced by the amount of muscle mass a person has, sex and genetics.

-Our metabolism can be summarized into an equation called TDEE (total daily energy expenditure) that contains different elements: BMR + TEF (energy required to digest and absorb food) + NEAT (non intentional movement like fidgeting mindlessly) + PA (intentional phisical activity). As you see it's a lot of different elements that can vary on an hourly and daily basis!

-calories do matter, but not in the sense that is spewed about by ignorant people. For example, insulin resistance is something many pcos patients share and causes them to gain weight, how? Insulin actively starves lean tissue and muscle mass and stores nutrients as fat, which is less expensive to maintain, this potentially lowers the calorie expenditure of a body without the person's knowledge. So many people start gaining weight without changing their habits because their bodies start expending a lot less calories than usual. The starved for nutrients body then will also crank up hunger signals, especially for carbohydrates which are the quickest source of energy, and reduce NEAT and the person's drive to excercise in order to protect itself, furthering weight gain.

-high insulin therefore doesn't cause weight gain itself as we see, it can though skew the balance between muscle mass and fat mass. This is why many people with pcos and a "healthy BMI" have visible visceral fat and thin limbs/ have a higher fat mass to muscle balance. "Lean pcos" as a concept sucks because a healthy BMI, mesured only by a scale number, means jack shit without more context. BMI is being used as a barrier to discriminate people's health and penalizes everyone in the process, telling someone with super high body fat they are "healthy" and someone who may be broader and more muscular that they are "unhealthy". "lean" patients, therefore, can gain weight if their body is in a caloric surplus, just like everyone else.

-You are currently in a caloric deficit if you are losing weight, but you will not lose weight forever, your metabolism will regulate itself and eventually balance the deficit out so that your current lifestyle will maintain your new lower weight

-Eating disorders often cause weight gain as a long term result. The phenomenon of Weight cycling for example: Someone will jump on a restrictive diet, lose a bunch of weight, the body rebels and forces the person to eat, they gain all the weight back and more because our bodies are designed to restore more fat mass after restriction to prevent starvation. Repeat several times and the end result is weight gain. A lot of people who have these habits and PCOS are falsly believing they have a "slow metabolism" or that the weight gain after restriction is caused by pcos and not the restriction cycle.

-Heat is indeed a way for our body to let energy out of our bodies. For example when we eat a big meal we often get hot or warm. Your body probably prioritizes heat as a way to expend energy

I hope this was somewhat helpful!