I myself have made some progress since I posted this question. But, I would be interested in your elaborating on a few things from your post:
First, this is the first time I've heard of Elmore 'Martha' Carin. How did you come by her, marriage records?
And could you tell me the names of the other children and your source?
The change of name is very interesting. Through the help of some native Quebecois, I've found that name changes after arriving in the U.S. weren't that unusual. I have a few theories on the reason in this case. One is that the script version of Xavier, with some rounded flourishes at the ends of the "X," could turn "Xavier" into "Oliver" in a mistaken transcription. I saw one hand-written record record of "Xavier" that gave me that impression. The other explanation is the Civil War. I don't think a French Canadian who came from a history of [[http://www.wikitree.com/wiki/Space:Scriven_Ancestry_Timeline|The Quebec Patriots Rebellion of 1837-38]] would be eager to enlist in a conflict that was not their own. Especially, since (to my knowledge) many Quebecois felt (rightfully) betrayed by Americans who didn't come to their aid in Quebec's own war for freedom. Finally, Oliver’s Naturalization papers say Oct 3, 1872, Carthage, NY, Why go cross state (130 miles for Black Brook, 200 for N. Adams, MA) for this? Was Francois-Xavier pretending to be Oliver for the sake of citizenship? (Identity “borrowing” from a relative that was about the same age?) Compare Fontaine-309 to Fontaine-435. They were both born in St. Remi within five years of one another. Coincidence?
Having said al that, it could be none of the above, something simple, plain, and uninteresting. One thing I do know is that records show Oliver Fontaine purchased a family plot at the Southview Cemetery in North Adams, MA right after his wife died. The tombstone has his wife's name AND Xavier (not Oliver) on it.