Is it permissible for a woman who has been divorced once and wears the niqab to return to her former husband, who is not observant but claims his desire to become upright and raise their daughter together, given her fear that her return might be impermissible and displeasing to Allah?

The Shari'ah encourages a woman to choose a husband who is religious and of good character. Marrying a fasiq (transgressor) does not invalidate the marriage as long as he is a Muslim. There is no impediment to your marriage to this man if he is a Muslim, especially if he shows a desire for righteousness; you might be a cause for his guidance.