Around 1990 I corresponded with a lady from Ontario who descends from the same Damien LeBlanc in whom you are presently interested. We were able to identify Damien as a son of Pierre LeBlanc and Marguerite Gauthier. Damien was born and baptized at Cap-Pelé on Nov. 15, 1823. The connection is made obvious from the listing of Damien and his second wife in the 1881 census at Néguac (Alnwick civil parish) next door to a house in which his eighty-year-old mother lived with his sister Domithilde and the latter's husband, Cyrille Comeau. Damien's father Pierre was the only known child of your ancestor Sylvain (dit Sailor)'s brother Joseph (dit Têtu).
Insofar as I know, there is no surviving record of Damien's first marriage, to Élisabeth (or Isabelle) McGrath (pronounced McGraw). Your second correspondent is right that Élisabeth McGrath was the daughter of Alexis McGrath and Anne Lirette who was born at Cocagne in 1825. This same Élisabeth was buried at Néguac on Nov. 5, 1878, aged "55" years.
I went through the Grande-Digue index and register, and I found eleven children recorded as Damien and Élisabeth's, all born between 1847 and 1868, just as you say. And I saw the errors that poor Father Gosselin entered. I do not think that the three children he baptized in 1858 and 1860 were the products of any sort of multiple birth. I think Father Gosselin simply made a mistake about Obéline's birthdate. It looks to me as though it should be 1859, and not 1858. That would mean that Damien and Élisabeth had Théophile in June 1858, then Obéline thirteen months later, and Scholastique thirteen months after that. That's a bit close, but they had already done something very similar between 1852 and 1854, with Luc in April 1852, Nazaire just a year later, and Alexis in July 1854.
The real problems arise, however, when one tries to trace this family in the censuses. As already mentioned, Damien is listed at Néguac in 1881 with his second wife. I have not been able to find him in the 1871 census at all, but here is what the 1861 census shows (in "Shédiac"):
"Damion White," 44; Isabelle, 40; Modeste, 17; Gertrude, 15; Marguerite, 13; "Sarah," 10; Sylvain, 8; Romain, 6; Marie, 4; and Philomène, 2.
Now that certainly doesn't match what we find in the registers very well, does it? Damien was really 47 and Élisabeth 45 at this time. Their daughter Modeste was only 13, Marguerite 10, and Sylvain 5. Yet it has to be them. Evidently there was a girl named Gertrude between Modeste and Marguerite; that is plausible enough. But who was "Sarah"? Was this really Nazaire ('Zaire = Sarah)? (I should note that I am basing this all on Tony LeBlanc's published transcription of this part of the census.) But how do we explain Romain, Marie, and Philomène? Are these Théophile, Obéline, and Scholastique? The connections are certainly not obvious to me with regard to these three. The only things that seem to match are the genders. As I said above, you have gotten into a tricky family here.