Is it permissible to marry an adulterous woman while fearing she may not safeguard her honor in the future, given the age difference and its potential impact on the first wife and children?

You must take the initiative to repent sincerely from this sinful relationship and from falling into immorality as a result of it, and cut off this relationship completely.

Regarding your marriage to her, if her repentance, rectitude, and good conduct are not established, we do not advise you to marry her. If you are in need of marrying a second wife and believe you are capable of being just, then seek a righteous woman. If you fear that marrying a second wife would cause harm to your family and children, then refraining from it might be better. The age difference between spouses is not a religious impediment.