Immaculate Conception

As a Lutheran of the Evangelical Catholic (some would say crypto-Catholic) persuasion, I've long held the Marian doctrines, as indeed did most of the great Reformers, including Luther and Calvin. Try telling them that Mary wasn't a perpetual Virgin, and you'd get a black eye! (Actually, Luther would have called you terrible names in public, and Calvin might've burned you at the stake. Oh, Calvin.) Anyway, the Immaculate Conception, of all the Marian doctrines, puzzled me the most. I found the evidence from Scripture and the Church Fathers quite convincing, but my question was this: if God found it fitting and proper to preserve Mary from all sin, original, mortal, and venial, from the very moment of her conception, why did He not find it fitting and proper to do that for us all? Special privilege of the Theotokos, perhaps? But the Immaculate Conception suddenly made sense to me when I saw it in the following light:

Christ received His flesh from Mary. She was the Body of Christ before Christ had a Body.

It follows, then, that hers was the Blood of Christ before Christ took on Blood. Thus, Mary is redeemed by the Blood of Christ, and was preserved from sin from the moment of her conception because she had His Blood from the moment of her conception. I think this obvious in retrospect, and hope I'm not straying into heresy unawares. As Origen begged, tear up my works if the Church judges me wrong.