When performing ablution (wudu), is the washing of the underside of the tongue from outside the mouth limited only to the part beneath the teeth of the lower jaw, or does it include the remaining parts of the underside of the tongue from outside the mouth?

Washing the area beneath the two jaws (the two bones from which teeth emerge) from outside the mouth is not required in ablution. This marks the lower boundary of the face, and there is no difference in this regard between a person who shaves and one who does not.