Depends on cause. Allergies, enlarged adenoids/tonsils, thumb/finger sucking, or a tongue anchored to the floor of the mouth are common causes. Heredity, neurological and muscular problems can contribute to this. See an ear nose and throat physician to evaluate the problem. Occupational/physical therapy can be helpful too.
Varies. It depends upon the age, reason for the tongue thrust, and to a great extent, the motivation of the patient. In my own experience, it is generally not easy to correct over the long term. However, especially if mild, it isn't necessarily something to be overly concerned about. There are much worse things and habits in life. Take the advice and recommendations from the doctor who diagnosed it.
Likely. Tongue rakes work well for this.
Good with practice. Nobody teaches a newborn how to swallow. Many combinations are possible, depending on the shape of the nipple used in feeding them when firstborn. Tongue thrust results when the tip of the tongue touches the teeth instead of the ridges of the palate. Since we swallow hundreds of times a day, this powerful muscle can move the front teeth creating problems. Therapists teach optimal swallowing.