To the OP, Christ is the only fitting Sacrifice because He’s both sinless and eternal. Similarly, an incarnated angel wouldn’t have been a sufficient Sacrifice. And Adam wouldn’t have been able to atone for Eve’s sins (or vice versa) at the point one but not both had sinned in Eden.
I’ve been in the Protestant church all my life and I never heard of that argument (maybe I just haven’t spent enough time with the Catholic bashers

). It’s an interesting argument. Jesus could give his life for our sins because he is sinless. But, if Mary is also sinless…
My take has always been that all men (including women) are sinful. I would say this is the typical Protestant objection. Only Jesus is called sinless in scripture and he manages that because he’s the son of God.
Catholics believe that Mary was preserved from sin miraculously, through application of the Blood of Christ across time. Likewise, in Heaven, we’ll be sinless, even though we don’t deserve to be by our own merits. So I think we can at least agree that it’s not logically impossible for Mary to be sinless, right? And when Paul says “all have sinned, and fall short of the glory of God,” he’s meaning all groups of people (that is, Jews and Gentiles alike), not every single individual. After all, he doesn’t mean Jesus, to point out the obvious.
I do have a logical objection to concluding that Mary is sinless because she’s the mother of Jesus. Why not conclude that Mary’s mother is also sinless because she’s the mother of the sinless Mother of God? This logic continues all the way back to Eve, who committed the first sin.
Great question. When we talk about this, we aren’t saying that a sinless person must have sinless parents, but that:
(1) Mary isn’t just the Mother of God, She’s the Tabernacle, the Ark of the Covenant. Look at the transition from Rev. 11 (which ends on the Ark) and Rev. 12 (which begins with Mary crowned in Heaven). So the OT Ark, because it was intrinsically Holy, was in a perfect shelter. That doesn’t mean every material for the Ark had to be in its own Ark,
ad infinitum. No, the Holy of Holies is in the Ark, so the Ark is to be purified. The Ark isn’t the Holy of Holies, so it doesn’t need its own Ark. Likewise, Christ is God, so He has an Ark in His Mom. She’s not God, so no need to keep the cycle going.
(2) Unlike Mary, Christ is taken entirely from Mary’s flesh and blood. Mary has DNA and everything else distinct from Her mom and dad. Jesus doesn’t. He purposely takes everything He uses to Incarnate Himself from Mary alone, which makes sense, given #3.
(3) Mary is the New Eve. Just as Paul calls Christ “the last Adam,” we see the early Church refer to Mary as “the New Eve.” For example, She’s called this in Against Heresies, which was written before the word “Trinity” was ever coined – it’s a belief dating back to the beginning of the Church. The idea is simple. Death enters the world through Adam, but subordinately, through Eve. Both being without original sin, but both choose to sin by eating of the fruit of the tree. The curse is reversed by Christ, and subordinately, by Mary. Both begin without original sin, and both resist temptation. Ultimately, instead of Eve taking the fruit from the tree to Adam, we see Mary at the foot of the Tree, with the Fruit of Her Womb on it, atoning for the sins of Adam with His Holy Blood.
All of this is prophesied in Genesis 3:15, which says that the “seed of the Woman” will crush the head of Satan. Normally, geneology is measured through the man, but the Woman in question is a Virgin, Mary, a fact Christ makes clear by calling Her Woman at the start and end of His ministry (that is, from the wedding at Cana to the Cross). Finally, Eve is the Mother of the Living (physically - see Genesis 3:20), while Mary is the Mother of the Living (spiritually - see Rev. 12:17).
Like I said, very good question. The Church Fathers have great insights on this, and I think we can trust their testimony. After all, if we can’t trust the Christians in 180 to have gotten it right, it seems hazardous to trust their canon of Scripture in 300 or so, right? And without it, how can we even speak of having a Bible?