What is the ruling on a woman who does not wear the niqab, and is she sinful for wearing the legal hijab, and does she deserve to enter Hellfire because of it?

Covering the face is obligatory according to the majority of jurists, and not doing so is a sin. However, for a female student of knowledge who has come to the conclusion that it is not obligatory, or who has been given a fatwa by someone she trusts, there is no sin upon her.