Is it necessary to re-fast a missed day (qada') after imagining a man's 'awra and discharging a thin, yellowish fluid without feeling lust? And what are the consequences of masturbating at night during Ramadan, unaware of its prohibition and the necessity of ghusl?

The aforementioned imagination does not affect the validity of the fast. The fluid that was discharged may be madhy (pre-seminal fluid), which does not affect the validity of the fast, or it could be from normal secretions. Practicing masturbation at night does not affect the validity of the fast, even without performing ghusl (ritual bath); because ghusl from janabah (state of ritual impurity) is not a condition for the validity of the fast. Rather, the concern is with the prayers you performed while in a state of janabah.