Otto, am I correct in assuming that Reformed protestants accept the Augustianisn construction of Original Sin as opposed to the Orthodox one of Ancestral Sin or the Sin of Adam? If indeed they do, isn't the Latin doctrine of the Immaculate Conception a theological necessity?
Though I'm not Otto, I shall (perhaps unfortunately) thrust my opinion in here.
As one other(presumably)Catholic contributor to this post said "God is not bound by your rules." And God would not be bound by some construct of ours such as "Original Sin." Anymore than God is bound by the construct of conception being between a man and a woman.
Why must we make two miracles of one? And by that I mean, why must we construct an Immaculate conception of Mary in order to attempt to explain the Incarnation? And even then coming up horribly lame in that attempt, whether with an Immaculate conception as part of it or without it.
I find the idea of God becoming flesh to be inconceivable (quite literally). I arrogantly suggest that anyone who does not, does not really believe in the Incarnation. I do not need to know the "how" of the Incarnation nor do I need to try to explain the "how." It is all gloriously beyond me and as such, any attempt to explain "how" Jesus was carried by a human seems to me to be a construct to fit our theological boxes rather than any sort of important idea about salvation or who God is.
As such, the idea of the Immaculate Conception seems to introduce more problems than it solves. And by adding another "layer" of miracles to the miracle of the Incarnation, it only serves to move the theological and, if you will, "logistical" problems of an Incarnation to the preceding generation. It does little to do away with those problems.
Of course, I could be completely and utterly wrong and welcome any thoughts to the contrary.