Is a person considered accountable after suffering from a severe mental illness that prevented him from praying, fasting, and performing purification, even though he understands speech but cannot fully take care of himself? And is the responsibility of making up for missed fasts lifted from him after entering a long coma?

If your mother owed days from Ramadan (definition) that she missed while she was conscious and accountable, and then she lived a period during which she was healthy and able to make them up but did not, then this is a debt upon her. Food for a poor person must be given from her estate for each day. It is permissible for you or someone else to fast on her behalf, based on the Prophet's (peace be upon him) saying: "Whoever dies and owes fasts, his guardian should fast on his behalf." This includes vows, missed Ramadan fasts, and other types of fasts.

However, if she passed away before she had the opportunity to fast, then there is nothing due from her. But if she recovered and was able to make up the fasts but did not, then it is legislated for her guardian to fast on her behalf.