Question 1: The fittingness of the Incarnation
Article 2: Was the Incarnation necessary to restore the human race from sin?
Quick answer: It depends on how you understand "necessary" (Thomas always distinguishes)
Why? Thomas says that a thing can be called necessary in two ways. The first is when it is absolutely essential, otherwise the goal can't be reached. In this sense, the Incarnation was not absolutely necessary because God could have devised other ways to save us.
Yet something can be necessary even when it is not absolutely required, but makes it easier to reach a goal. He gives the example of using a horse on a journey. (Thomas traveled a lot through Europe, mostly on foot. We can imagine he probably often wished he had a horse!)
The Incarnation is necessary in this second way. It helps us in two ways: first by making it easier for us to attain good, and second by making it easier for us to avoid evil. In this post I'll only speak about the first way, since this is a longer article than usual.
Thomas gives five reasons why the Incarnation helps us to attain good more easily. He quotes Augustine for each of them, which shows how much he looked to Augustine in formulating his own thought.
1) Our faith is more certain because we have it on the authority of God himself speaking to us in Jesus Christ. "In order that we might journey more trustfully toward the truth, the Truth itself, the Son of God, having assumed human nature, established and founded faith."
2) Our hope is more certain because it is greatly strengthened, as Augustine says, "Nothing was so necessary for raising our hope as to show us how deeply God loved us. And what could afford us a stronger proof of this than that the Son of God should become a partner with us of human nature?"
3) Our love is increased because of the Incarnation since as Augustine says, "What greater cause is there of the Lord's coming than to show God's love for us?" And he adds: "If we have been slow to love, at least let us hasten to love in return."
4) Jesus gave us a model for right living: "God was made man, that He Who might be seen by us, and Whom we might follow, might be shown to us."
5) Through the humanity of Christ we are enabled to participate in God's own life through grace: "God became a human being, that human beings might become God."
That last quotation from Augustine was a favorite one of the Fathers of the Church, and goes back as far as Irenaeus (d. 202).
Spiritual takeaway: Each of the five reasons and the quotes from Augustine would make a fruitful meditation. I can't add anything to that!