Is giving a young maid a spoonful of chili to make her tell the truth considered a major sin, and are the methods used to discipline her, including beating, wrong, knowing that she steals, swears falsely, and lives in a corrupt environment?

Your kindness towards this girl and your endeavor to rectify her morals are commendable in Islamic law. Therefore, you must show her mercy and compassion, refrain from humiliating her, and try to convince her of good morals, gently deterring her from bad ones. You must teach her matters of religion. You are not permitted to punish her with the حد (prescribed punishment) for theft, as its enforcement is reserved for the Muslim ruler. However, it is permissible to discipline her with a beating after exhausting other means, and to the extent you would beat your own child.