Which takes precedence: paying the usufruct amount from a Riba-based loan or performing Hajj?

Your mother only needs to offer sincere repentance, turn to Allah, and multiply her good deeds. She does not need to give away any of her money, as she does not possess unlawful wealth that she needs to dispose of. The usurious interest was taken from her, not paid to her. What you mentioned about disposing of unlawful money applies to someone who received usurious interest or acquired unlawful funds. If your mother is able to perform Hajj, she should hasten to do so.