Does what transpired in the speaker's mind—a low-voiced conversation about her husband with a girl—constitute slander (qadhf) that necessitates sin, or is it among the excused inner thoughts, and is a person held accountable for their inner thoughts?

Your statement to your husband, "When he comes from outside, I will ask him to tell me everything," is not slander. Inner thoughts are not held against a person unless they lead to a spoken word or an action, based on the Prophet's (peace be upon him) saying: "Indeed, Allah, the Mighty and Majestic, has forgiven my Ummah for whatever they think in their hearts, as long as they do not act upon it or speak of it." However, if one utters the words, even secretly, they will be held accountable for it. If a wife becomes aware of a wrongdoing by her husband, she should advise him kindly and remind him of Allah.