1. Does wearing a corset have any disadvantages?
The wearing of a corset for extended periods of time to maintain or reduce waist size is called tightlacing. It is commented (though nowhere I can cite) that to effectively tightlace you need to use a Victorian-style steel/bone corset as modern elastic/rubber/plastic corsets don't exert enough force.
A typical tightlacing training routine begins with the use of a well-fitted corset and introduces very gradual decreases in the waist circumference. Lacing too tight too fast can cause extreme discomfort and short-term problems such as shortness of breath and faintness, indigestion, and chafing of the skin if a liner is not worn.
The diminished waist and tight corset reduce the volume of the torso. This is sometimes reduced even further by styles of corset that force the torso to taper towards the waist, which pushes the lower ribs inwards. Internal organs are moved closer together and out of their original positions in a way similar to the way that a pregnant woman's expanding uterus causes the organs to be displaced.
The volume of the lungs diminishes and the tightlacer tends to breathe intercostally – that is, with the upper portion of the lungs only. Due to the lower portion of the lungs being used less there is often a stereotype of mucosal build-up there; a slight and persistent cough is the sign of the body trying to clear this.
For more information on the effects on individual organs please see this Wikipedia Commons article.
Feel free to google any of the above, its simple biology. You cant magically break the rules of thermodynamics or human biology by squeezing your waist.
2. What if I wear it for 8 hours a day?
The same as above still applies.
3. Does a corset affect the skin ( or as I read the fat under the skin )?
In a way, it moves it around which does 'affect' it.
It does not build muscle and may lead to atrophy of your abdominals due to them not being used to stabilise you (as you rely on the corset).
For more information: https://en.wikipedia.org/wiki/Tightlacing
There are some benefits to tempory waist restriction, when power/weightlifting a tight belt can be used to brace ones core against so that the persons posture is kept more rigid at higher weights. It can even help develop abdominal strength by giving you resistance to push against