Is a daughter's speaking about her non-Muslim mother's ill-treatment of her due to her conversion to Islam considered backbiting, given that she knows her mother is influenced by American media?

What a Muslim woman experiences in terms of harm after choosing Islam is natural. Backbiting is mentioning others with what they dislike, and it is permissible for seeking advice or complaining about an oppressor, like the hadith (definition) of the woman who complained about her husband to the Prophet, peace be upon him, or the man who complained about his neighbor. So, what you did in complaining about your mother to seek advice is permissible, and you are not accountable for what you said.