Is she considered an unbeliever, one who wished that the period of menstruation were limited to specific days to avoid confusion, intending that what Allah has decreed is not the best?

A woman wishing for relief from menstruation does not constitute disbelief, as her wish might be to eliminate the confusion and doubt it causes her. Scholars have stated that the fundamental principle is that a Muslim remains in their state of Islam with certainty, and they do not exit it except with certainty. It is not obligatory for a servant to be pleased with every decree (al-maqdi), but only with the Divine Decree (al-qada'). Al-qada' is entirely good, just, and wise, and we are pleased with all of it. However, al-maqdi (that which is decreed) is divided into what one should be pleased with and what one should not be pleased with.