Is the emission of pre-seminal fluid by a girl upon meeting her fiancé—without a valid شرعي ( Shar’i ) contract—considered a sin that necessitates expiation, given her unsuccessful attempts to control it, and the conversation being limited to serious matters?

A woman's speech with non-mahram men—including a suitor—is not permissible unless there is a need, and provided there is no fear of temptation (fitna) and no softness in speech (khudu' bil qawl). If your conversation with the suitor arouses temptation and incites desire, then it is not permissible, and you must avoid it. You can appoint a relative to speak with him. However, if that is merely an illusion or a whisper (waswas), then it is not forbidden to speak with him to the extent of the need, provided the aforementioned conditions are met.