Is it permissible to pay the costs of treatment for the deceased from her estate, whether she bequeathed it or not, or is it obligatory for the well-off heirs to pay it from their own money?

The costs of a woman's medical treatment are paid from her estate, as it is a debt upon her. The heirs are not required to pay it from their private funds. Then, whatever remains of the estate is divided according to Islamic law.