Is it permissible for a woman to uncover the area between the lower jaw and the neck (which is the space under the chin) during prayer, and is this area considered part of the chin?

The face is what one confronts with, and the area below the chin is not considered part of it and must be covered during prayer.

As for the chin itself, it is part of the face and must be washed during ablution, according to the Almighty’s saying: {O you who have believed, when you rise to [perform] prayer, wash your faces}.

Linguistically, the chin is the meeting point of the two jawbones from their lower part, and it is also used to refer to the entire face. The vertical boundary of the face extends from the hairline to the bottom of the chin, which is the end of the jawbones.