Is an elderly woman ignorant of her religion and not performing the Salah obligated to learn it, and will Allah accept her Hajj despite her ignorance, or is it preferable to perform Hajj on her behalf?

If a woman is sane and discerning, she is obligated to fulfill the duties of Islam, and nothing is waived for her except for an excuse. Learning the rulings of prayer is something achievable, so she must learn what she can. A woman's help to people will not benefit her in the afterlife if she abandons prayer, because one who abandons prayer is a disbeliever, and a disbeliever has no reward in the afterlife. The Hajj of this woman is not valid, nor is deputizing someone for it, because of her abandoning prayer. If she prays, then Hajj becomes obligatory for her if she is able. If she is able only financially, she should delegate someone to perform Hajj on her behalf.