So...God could prevent Mary from being tainted by original sin, but he couldn’t do that for Jesus?
But if His intent was that Mary would have a role in salvation history beyond her natural role as a human being then His plan would likely provide a special consideration for her that would be far beyond even what He would bestow upon the greatest of His saints.
Something else that's worth noting is that Mary occupies a very special place even from a purely natural/scientific perspective.
If Christ was conceived by the Holy Spirit and had no "natural" human father, then His human form would contain a genetic code (DNA) that was identical to Mary's. I would ponder on that for a moment and think about the possible ramifications of that salient fact.