Will a mother be held accountable for her period of not praying due to severe illness and depression, and can this be expiated for, and will the illnesses that afflicted her intercede for her on the Day of Resurrection?

If the mother was unconscious during her illness, then prayer is not obligatory upon her due to the lifting of responsibility. However, if she was conscious, then prayer is obligatory upon her according to her ability and it is not waived from her. It is not prescribed to make up the prayers she missed after her death. It is prescribed to supplicate for her and give charity on her behalf. Illness is an expiation for sins, as stated in the Hadith: "No fatigue, nor disease, nor sorrow, nor sadness, nor hurt, nor distress befalls a Muslim, even if it were the prick of a thorn, but Allah expiates some of his sins thereby."