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The Catholic petitions Mary, who petitions God for the favor. So the ultimate source is God. But then, there is a different stream of belief which says that God will grant anything that Mary asks, because he cannot refuse her. And if God cannot refuse Mary, then she does have functionally divine power.

And Mary has no will outside of the will of God. So there is no thing that God would "have" to do that God didn't want to do, just because Mary "wanted" it.

It's all somewhat circular.

she does not control to whom they go

Then why pray to her for graces?

Popular devotions such as this clearly suggest that Mary distributes the graces.

Yes, that's her job. She distributes the graces. But she does nto operate independently of God.

There is no clash of Wills. No one is getting any graces from Mary that God didn't intend.