According to the best historical info I can find on the Duloe site, the two smallest "original" stones are only incorrectly placed fragments of the larger stones during it's clumsy restoration.
apparently it was only officially "discovered" in 1806, after a hedge which ran through it was removed, previous to the restoration all of the stones were actually fallen. has there actually been a good(i.e modern) archaeological survey done of this site, the most recent i could find documented was from 1882 - which doesn't particularly me inspire much faith, given victorian archaeological knowledge and techniques.
So perhaps the new stone IF it was found lying around locally is just as much right to be there as the two smallest stones anyway....lol.
As for the fairies, I know just what you mean....the things some people leave at ancient sites( reminds me of a post I read here or somewhere of a 3 foot carved wooden leopard being left in one of the side chambers at West Kennet!!), however I take the view that it's certainly better than setting fires (including burning tea lights in alcoves) or daubing paint or chalk everywhere!.
My take on it is....it's just basic littering, not as bad as vandalism but still bad.....if it makes people happy to pick a few flowers and leave them at sites....fine....they bio-degrade quickly. the junk like the pixies and the aforementioned leopard don't.
And whilst this may seem a harsh attitude.....what would happen if everyone came and left junk at these sites?