
   Yes, Mary is fully human.  However, that does not imply that she was
just as subject to sin as we are.  Catholic doctrine says that man's
nature is good (Gen 1:31), but is damaged by Original Sin (Rom 5:12-16).
In that case, being undamaged by Original Sin, Mary is more fully human
than any of the rest of us.

   You ask why God cannot "repeat the miracle" of Mary's preservation
from Original Sin.  A better way to phrase it would be "why _did_ He
not" do it that way, but you misunderstand how Mary's salvation was
obtained.  Like ours, the Blessed Virgin Mary's salvation was obtained
through the merits of the Sacrifice of Christ on the Cross.  However, as
God is not bound by time, which is His creation, God is free to apply
His Sacrifice to anyone at any time, even if that person lived before
Christ came to Earth, from our time-bound perspective.  Therefore,
Christ's Death and Resurrection still served a necessary purpose, and
were necessary even for Mary's salvation.