You are correct in showing the use of this passage to support the order of Mary's miracles. I used the chronological order of the miracles to show the saving of Willie as the first miracle. Dunstan does say that Willie's death is Mary's second miracle and refers to her saving of Joel (the tramp) as her first. That said, Dunstan also begins to think about Mary saving him from the snowball as a miracle when looking back on her life. If this is the case, then this would be her first miracle.
Like much of literature, interpretation is everything. In accordance with time, the saving of Willie (or the snowball) comes first. When Paul and Dunstan are talking about Mary being a saint, Dunstan says that her saving of Paul was her first miracle. It really depends upon if one is analyzing the chronological order of her miracles or Dunstan's numbering of them.