How could Abraham or Moses stay the hand of God when they begged for the lives of the people under their care/authority? The same way Mary can–by understanding that God wants us to intercede for one another. God loves each and every person. He doesn’t wish to bring judgment on anyone. By allowing himself to be appealed to through another human being’s intercession he is making it clear that his will is to save not damn as many people as possible. It’s a positive thing not a negative thing that St. Louis de Montfort is conveying to us. God is appeased (because he wills to be appeased in his love for us) by all our prayers for one another, but especially those of Mary who is the greatest of all the Saints. I hope that helps.
Not just because they intercede for us but rather because God wills to withhold his justice by means of his mercy reflected in the prayers of the Saints. You see it all begins and ends with God. He created the Saints by redeeming us in Christ (we’re all supposed to be Saints–holy) and he accomplishes his will in the Saints through his own grace. So, all is done in God, by God, and through God.
Yes. God is enacting his will through the Saints, who are the Church Triumphant while we still on earth are the Church Militant–working in concert with the Church Triumphant to do God’s will.