Would divorcing a wife who had a past of [illicit] relationships and reverted to former behaviors, then repented and committed to prayer, the Quran, and household duties, be considered an injustice to her, or a relief for her from the husband's doubt?

Divorce is permissible out of necessity and disliked without necessity, according to the majority of scholars. If you are upset with your wife due to what she has done and wish to divorce her, this is permissible. However, if your wife has repented from what she fell into, regretted it, and her condition has improved, then you should not divorce her. It is also not permissible for you to taunt her about her sin or to seek out her missteps. Rather, you should live with her kindly and conceal her past sins, for repentance erases what came before it. Allah Almighty says: (And live with them honorably. If you dislike them, it may be that you dislike a thing and Allah brings through it much good.)