Mary's Fiat: Freedom for Surrogacy?

I consider the role of Mary in Barth’s theology as Mary’s acceptance of the virgin birth serves as a prime example of God’s freedom alongside human freedom to acknowledge God’s work granted through grace. How does his identification of her as the climax of God’s election of Israel relate to the particularity of her embodied experience as Theotokos? Does Barth’s treatment of Mary’s role in the incarnation discount agency over her body, especially if one accepts the logical priority of election over triunity? Does Mary’s humanity as eternally presumed for the identity of the Son give Mary any autonomy over her pregnancy? Is Barth’s treatment of Mary, especially in light of Mitzi Smith’s work on Mary as doule (slave), create a problematic surrogacy that warrants critique similar to what Dolores William’s directed toward substitutionary atonement?