Is it obligatory to tell one's mother about a past theft of her gold, knowing that she still curses the one who stole it and thinks it was the maid, and the money has been returned to her without her knowing the perpetrator? And is it sufficient to conceal the act, repent, and show filial piety to seek her رضا (pleasure/satisfaction)?

If someone unlawfully seizes jewelry and then consumes it, they must return its value. If the questioner returns the value of the gold to her mother, her conscience will be cleared of this right. However, she still needs to repent to Allah and strive to be exceptionally dutiful and kind to her mother as atonement for her past actions. She is not obliged to inform her mother that she was the one who took it if she fears that this would negatively affect their relationship. She should advise her mother not to accuse innocent people without proof, as that is a grave sin, based on the Almighty's saying: "And those who harm believing men and believing women for [something] other than what they have earned have certainly borne slander and manifest sin," and the Prophet's (peace be upon him) saying: "Whoever says about a believer what is not in him, Allah will make him dwell in the mire of al-khabal until he retracts what he said, and he will not be able to retract it."