Is what a menstruating woman sees in a dream not considered a legitimate vision, and thus not to be taken into account, due to her state of impurity and her sleeping without ablution, and therefore it falls under the category of Satan's dreams?

Purity (taharah) is not a condition for the validity of a dream. A disbeliever may see a true dream, as happened with Joseph and the king. However, sleeping in a state of purity makes the dream more truthful and keeps the devils from tampering with it. It is recommended that the dreamer be truthful, sleep with ablution (wudu') on their right side, and remember Allah.