We have two levels of goodness here: the righteous yet afflicted with original sin stock producing what would have been a righteous yet afflicted with original sin Mary, and the Divine Goodness that gives birth to man Jesus. If the Father had made Jesus out of clay like Adam, there would be no questioon of how, -- but it pleased the Father to use a human mother. She is then both a culmination of the human Jewish stock, and a tabernacle of the Divine Word. This supports the immaculacy of Mary; it is logical that the immaculacy was imparted onto her at her conception.
So, "eating His flesh and drinking His blood" can mean something other than the physical acts of eating and drinking
Speculate you. The context of John 6 is firmly against any symbolic interpretation of the Eucharist. The Good Thief episode merely explains that one should "eat the flesh of the Son of Man" if he is able to do so, and the Good Thief was not.