If the plates "kind stand up like angel wings" I would make one stand alone quarter plate and back it (by putting it wrong sides together with a white fabric that matches and turning) and applique it on in such a way that it does stand out a bit. Surely it will look more intentional then the missing piece and will most definitely make the mistake not noticible to anyone.
It really is quite obvious that the upper left hand corner snowflake is missing an arm. Even if you had not mentioned there was a mistake I would have noticed it and wondered why the pattern was that way.