Here is what I have about Laurent Ducharme and Marguerite-Amable Métivier's children.
1) Louis, born and died in 1754 in St-Laurent, Montréal, QC
2) Joseph-Laurent, born in 1758 in Michillimakinac, like you wrote it. He must be the one who died on Dec. 29, 1793, in Sault-St-Louis, near Montréal, at 35 years 7 months. Unfortunately, parents'names are not given in the burial act.
3) and 4) Twins Louis and Pierre Augustin, born in 1760, as you wrote it. Pierre Augustin died on Aug. 10, 1769 in Montréal. I have no further information on Louis.
5) Marie-Marguerite, born Aug 19, 1763, in Michillimakinac. She married Joseph Roi-dit-Portelance on May 23, 1785, on Sault-St-Louis, QC
6) Marie-Madeleine, born Jan 17, 1766, in Montréal. She married twice in the Montréal area.
Now Marguerite-Amable Métivier died on Nov 18, 1779, in Montréal. She was around 49 years old. Her husband Laurent died on Jan 13, 1798, also in Montréal.
So we can get to a few conclusions:
1) Of the fours sons to Laurent and Marguerite-Amable, only Louis (one of the twin) maye have lived long enough to have children. He may be Cuthbert's father, but he would have been old (67 years), and it' rather hard to believe, since Cuthbert's said is father was Jean-Marie.
2) There is some space for more children to Laurent and Marguerite-Amable, for instance in 1756, and from 1768 to 1770 or 1772. A Jean-Marie could have been born for instance in 1772; then he would have been 55 when Cuthbert was born.
3) Laurent may also have some other children from Indian Women, which was not unusual, to say the less.
4) Since Cuthbert said that he was the son of Jean-Marie at the grandson of Laurent we could take that for granted, at least till we can find some evidence that it's not true. Likely, proofs will be hard to find in either way.