Is what the menstruating woman in a state of indecision (mustahadah mutahayyirah) did correct, when she considered herself menstruating for seven days from the beginning of Ramadan, then performed ghusl and fasted for the rest of the month?

A woman experiencing irregular or undifferentiated istihada (non-menstrual bleeding) should consider six or seven days as her menstrual period, based on investigation. She should perform ghusl (definition) (ritual bath) after this period ends, then perform wudu (ablution) for each prayer. What you have done is correct.