Is not sending money to one's deceased mother – due to its unavailability or the husband's refusal – considered an act of disobedience (ʿuqūq), knowing that the daughter was living at her husband's expense?

Your request for money from your husband for your mother is not an act of disobedience (ʿuqūq), as it is not obligatory upon you nor upon him to do so. His donation would be an act of kindness from him. You are not obligated to spend on her as long as you are not wealthy. You can show devotion to her after her death through prayer, charity, maintaining ties of kinship on her behalf, and honoring her friends.