it is harmful because you cannot breathe normally, the pressure rises, plus a lack of oxygen, extra tension, the stomach should be relaxed, and strain only during physical exertion. Usually, the belly of the neck, shoulders, a person squeezes when he is stressed, and can walk like this for years due to stress, and over time can earn a bunch of chronic sores and disruptions in the body ...
No, because the body does not experience the proper load during static muscle work. Constantly keeping your abdominal muscles in tension will only give you the opportunity to keep your stomach in tension even longer. Example: pistol shooters, their main exercise is to hold the hand in the same way when shooting, it is definitely the time and even the shooters - Olympic champions do not have huge pumped up arms, tk. this is static tension, it does not give muscle volume or burns fat, and therefore the stomach will not become "flat".
Yes, in this way the transverse abdominal muscle works well, which holds intra-abdominal pressure. Its tone contributes to that very "flat stomach". I would advise doing the "vacuum in the abdomen" exercise on an empty stomach, an effective thing.
Sucking in your belly is really great. This is one of the main micro-exercises that we can do on our own anytime, anywhere. Moreover, it is not necessary to strive to suck in the stomach all the time. It is enough to do several sessions a day - 20-30 times: strained, fixed for a few seconds, relaxed. This exercise will help to remove a few centimeters in the waist in a couple of weeks.