Is it permissible for a brother to interfere in all of his sister's affairs, prevent her from completing her studies and marrying whom she wishes, and pry into her private matters, all under the pretext of holding her accountable for her past, uncommitted lifestyle?

May Allah reward you for your inclination towards repentance and righteousness. If your brother holds you accountable for sins you committed in the past, he is wrong. He has no right to prevent suitable suitors from reaching you, and if he does so, he is obstructing your marriage (عاضل), and you have the right to refer the matter to a Sharia judge. He is not permitted to prevent you from continuing your studies without a legitimate Sharia reason, and you have the right to dispose of your money as you wish within the bounds of Sharia. He is not allowed to pry into your private matters without your permission, and it is even more emphasized to prevent him from doing so if it involves spying.