I think this question is something about the wisdom and God’s Devine plan and the Virgin Mary is the handmade of God included in His wisdom and Devine plan.
Regarless whether Adam had sinned, Christ still would have incarnated according to St. Thomas Aquinas.
St. Thomas Aquinas (in his Summa Theologica) states that God’s power is not limited and therefore God could have become incarnate even if sin had not existed. However, St. Thomas believes that if man had not sinned then the Son would not have become incarnate. In the Sacred Scripture the sin of the first man is assigned as the reason of Incarnation, it is more in accordance with this to say that the work of Incarnation was ordained by God as a remedy for sin; so that, had sin not existed, Incarnation would not have been (Summa Theologica, Part III, Question 1, Article 1).
Saint Albert the Great – the master and teacher of Aquinas. Saint Albert teaches that the Divine Logos would have become man even if man had not sinned:
“I believe that the Son of God would have become man even if there had been no sin…Nevertheless, on this subject I say nothing in a definitive manner; but I believe that what I said is more in harmony with the piety of faith.”
“Credo quod Filius Dei factus fuisset homo, etiamsi numquam fuisset peccatum…tamen nihil de hoc asserendo dico : sed credo hoc quod dixi, magis concordare pietati fidei.”
– St Albertus Magnus, III In Sententiarum d. 20, a. 4
Saint Francis de Sales and Saint Lawrence of Brindisi (both official doctors of the Catholic Church) also held to the thesis that the Incarnation of Christ would have occurred even if man had never sinned.
They reason there is nature, grace, and glory. God’s assumption of a human nature entails that Christ’s created soul beholds the beatific vision from the moment of its existence. By participating in this reality – the participation of the created in the beatitude of God – angels and men are also able to participate in the divine beatitude.
In this scenario, glory and beatitude depend on the Incarnation. Now if sin was the sole occasion of the incarnation, then sin was necessary – yet this is blasphemy. This also entails that Christ’s humanity is conditioned by rebellion and sin.
We must also ask a few more questions.
Is the light of glory granted to us in and through the created soul of Christ or not? If the light of glory for beatitude is granted to us in and through the soul of Christ, then it seems that the incarnation of Christ is necessary for the beatitude of the angels and the beatitude of humans. If that is the case and if God willed to share His divine beatitude with angels and humans, then the incarnation would have happened whether there was sin or not.
Creation is contingent. The Incarnation is contingent. However, might the creation be ordered to the incarnation? Is not creation created in and through and for Christ? So then, might the goal and purpose of creation be the incarnation and the sharing of beatitude with creatures?