Is it obligatory to make up and offer expiation for someone whose sexual desire was aroused during a Ramadan day, with a discharge of fluid about which they are unsure whether it is madhy (pre-seminal fluid) or mani (seminal fluid)?

What you saw was madhy (pre-seminal fluid), not maniy (seminal fluid), and your fast is valid according to the preponderant opinion. You are not required to make up the fast nor offer expiation. If the fiancée is a non-mahram, then it is not permissible to converse with her in speech that contains suspicion, and you must repent to Allah for what you have fallen into, for sins committed in Ramadan are graver.