In 2015, my father died and my mother has dementia. How do I sign joint tax return?

I have POA for my mother who has dementia. My father died in 2015. When I file the 2015 joint return for my mother and father, I will:

1.  write on the top Form 1040 “Deceased,” my father's name and date of death;
2.  I will attach a copy of the POA for my mother; and
3.  on Form 1040 page 2 sign my name followed by “POA.” Or do I sign my mother's name followed by my name and POA.

As to my father's signature line, do I leave this blank or do I sign as executor and attach the court document assigning me as executor?