Yeah, that was somewhat unclear....
I believe that your comments about Anselm to be pretty much spot on. I believe, in many cases, that Aquinas took was Anselm said and ran with it.
Perhaps the most difficult element in the traditional Catholic teaching on original sin is the claim that the very guilt of Adam’s sin has been passed on to the entire race. It was Augustine who, in the course of the Pelagian controversy, first drew this conclusion from St. Paul’s discussion of sin, particularly in the letter to the Romans. Augustine ultimately attributed the transmission to the agency of concupiscence, a less than happy solution. When he came to treat the issue in his
Summa, Aquinas shifted the agency from the impetus of concupiscence to the will of the first man, a move of marked importance in the Augustinian tradition.
Both theologians see the unnatural concupiscence inherent in carnal generation as contributing to the propagation of original sin. For Augustine, this itself is the method of original sin's propagation. In violation of the will's rule over the soul, generation occurs and is thus subject to original sin. For Aquinas however, this is only the method by which Adam's will moves his descendants, and thus makes them subject to original sin as participators in his nature. Thus, the distinction between the will (Augustine) and the nature (Aquinas) as to the location of the prime subject of original sin.
However, before getting into a lengthy discussion it is also important to note (which is far too long a discussion for the OP) the somewhat divergent theological views between Augustine and Aquinas on "original justice" - which underpin their somewhat different views on Original Sin. I'm pretty much ignoring Anselm here - while his views are interesting - it is Aquinas' "completion" of Anselm's views which are dominant in the Western Church.
When reading all the various documents one does not find Anselm using Augustine’s term for guilt,
reatus, though that is the word that the Council of Trent will use, perhaps pointedly, in its formal teaching on original guilt. Instead, Anselm speaks of the
culpa, the fault or blame that each of us bears. The sin of Adam has made all of us accountable. We are all responsible for the sin committed at the origin of our nature, liable for the debt (
debitum) which that sin incurred, and yet, to a person, incapable of paying the debt.
Aquinas' views can obviously be found in the Summa. However, the best explanation of Anselm is not found in Why God Became Man, but rather its sequel: On the Virgin Conception and Original Sin.