As regards your appeal to Christain creed, your example illustrates the problem. First, as you are aware I'm sure, the creed is widely regarded as an interpolation that interrupts the discussion. Since we have reason to be at least skeptical of it's being Paul, and more importantly it doesn't define anything bearing on this topic, it doesn't form much of an argument regarding the theory of ransom.
But since you brought it up, notice that the creed contradicts the Gospel/Acts tradition and Galatians on a number of points. It is for this reason many scholars, even highly critical ones date (at least the first 2 verses of) the creed to a period before the Gospels and introduced into Paul fairly early.
R.Price has made reasonable arguments that the entirety was introduced post-Acts and was a result of a merging of rival Petrine and Jamesian sects' creeds.
Consider carefully reading this somewhat long article: Apocryphal Apparitions by Robert M. Price (mindvendor.com)
Something else I find glaring about the Creed is repeated appeal to "according to the scriptures". What scriptures? if the Gospels are implied, we have come to end of the discussion, it is clearly a late interpolation. If they refer to the Tanakh OT, then more difficult questions arise. Most assume the "scriptures" meant the Jonah story or possibly Hosea 6:2. This itself supports the position that the Christ story in all aspects were drawn from OT and seen through eyes of faith and eisegesis. This was subsequently allegedlyy confirmed through apparitions and visions of James the Just (not yet brother of Jesus) and Cephus. (not yet the rock of proto-catholic tradition). The 12 reference creates another set of issues I don't have time to discuss here.
So what we might reasonably conclude is that elements of the creed date to a period before the Gospel traditions. Likely the creed as it reads today is a patchwork of these early elements with later addenda referencing 500 and Paul.
