Harriet Tacey...Henri Tessier/Petronella Droomers Henri Tessier...Prosper Tessier/Henriette Charron dite Ducharme Prosper Tessier...Simon Tessier/Marie-Charlotte Jacques Simon Tessier Jean-Baptiste Tessier/Therese Huppe dite Lagroix/Lacroix Jean-Baptiste Tessier-dit-Lavigne...Jacques/Marie-Anne Sauliquin Jacques Tessier-dit-Lavigne...Ignace Tessier-dit-Lavigne/Marguerite Lucier Ignace Tessier-dit-Lavigne...Urbain Ignace Tessier-dit-Lavigne/Marie Archambault etc.
So we do have the first 3 North American lines in common. From the looks of it, your first American was about 3rd or 4th cousin to my first. I'd also venture to guess that Mary Grossman had a lot more to do with changing the name to "Casey" than anyone else. When the Brits married the French, there was a natural tendency to distance themselves from the "dirty" French-Canadians. Casey is a very Irish name and they might have used that to sound less French. Just speculation, but the coincidence is pretty striking.