Must one make up for the fasting days that coincided with the expected menstrual period, or make up for all the preceding days during which light bleeding occurred and its cause was not determined?

This blood is metrorrhagia (istihada), not menstruation (hayd), because it cannot be combined with the previous bleeding nor can it be considered a separate menstrual period. Your fasting during those days is valid. What you did by not fasting during your usual period is what is required, as a woman experiencing metrorrhagia refers back to her regular habit. You are required to make up for the days you broke your fast during your usual period, not for the days you fasted during the time of metrorrhagia.