Yes, Marguerite and Celeste Fournier are the same person, wife of François Soucy. The name Marguerite appears only on the mariage record of Henri Soucy and Adèle Laroche in 1857 but the priest obviously made a mistake. On the christening record of Henri Soucy, his parents are correctly mentionned as François Soucy and Céleste Fournier.
By the way, Henri Soucy left Canada about 1879 and settled with his family in Haverhill, Mass. where Adèle passed away on Feb 15, 1911. Henri was still living there in 1920 in his late 80s.