Imo weight class should be determined by an average of a bunch of weigh-ins for weeks or months before an event. Then, on the day of the event, allow some tolerance over the max weight but only if the average stays under. If the average creeps up into the tolerance zone, then they go up a weight class. Make the weight classes based on healthy habits rather than basically cheating in an unhealthy way to make weight a day before, then bulking back up to whatever weight for the actual fight.

Not that I have any stake in any of that, so my say isn't really relevant.

Resistance training. When on a calorie deficit while doing weight training, you will lose less muscle. You'll still lose some, but your body will try to rebuild the muscle using proteins and stored nutrients.

It is complex, precise dietary requirements vary person to person (even mouse to mouse but less for lab strains). As a generality women have lower caloric requirements, men higher. Age, height, weight, activity level, genetics, etc all have impacts.

The timeline of caloric restriction matters. Muscle breakdown occurs first because muscles store glycogen which is a much faster energy source than fat. Fat breakdown can go on for longer as it has a higher density of energy. Simple short-term reduction in calories thus results in muscle before fat. Higher protein intake mitigates or prevents muscle loss in longer-term diets compared to lower protein intake as the muscle cells are used to process/use the protein. Exercise is the best at keeping or growing muscle as it turns them from energy providers to energy users.