Is a wife required to make up 28 days from last Ramadan due to prolonged, intermittent bleeding that she considered metrorrhagia, or is her fasting for 15 days, based on previous fatwas, valid?

The basic principle is that any blood discharged is menstrual blood. If it continues for more than fifteen days, then it is irregular bleeding (istihada). Your wife erred by only waiting seven days at the beginning of Ramadan, as the menstrual period can increase or decrease. If the bleeding exceeds 15 days for the first time, it is istihada. However, before that, it is possible that the menstrual period has increased, and a woman is not considered to have istihada until the bleeding surpasses 15 days. If it becomes clear that the woman has istihada (because the bleeding exceeded 15 days), and she fasted the days beyond her usual period, then her fasting is valid, because it became evident that the blood was istihada and not menstruation.