I am afraid we still have not solved the "Mathurine Guillemot riddle" !
But before I address your last message, a word about GUILLEMOTs and GUILLEMETs. Both patronyms are attributed to Mathurine in that last message of yours. But in reality, GUILLEMOT and GUILLEMET refer to two entirely different sets of people, and are not interchangeable. It is normal, though, that they would at times be "confused" (just like HUBERT and HEBERT, also very different and distinct).
I think you can keep assuming that Mathurine was a GILLEMOT and not a GUILLEMET.
I find it interesting that her name (i.e. Mathurine GUILLEMOT) and that of AUFRERE would appear together in a record having to do with "Jean-Baptiste LALANDE and Dame Charlotte MARCHAND his wife".
Indeed, that Jean-Baptiste LALANDE was in fact a GUILLEMOT dit Lallande, son of François GUILLEMOT dit Lalande and Madeleine DUPONT. He was first married to Catherine OUABANAKIKOUE (also spelled OUABENAQUIQUOY), an Indian woman. The precise date of that marriage is not known, but it is infered to have taken place "in about" 1721 in Kaskaskia, where a son was born to them in 1722 (they also had a son in Kaskaskia in 1723, and a 3rd one at Fort de Chartes in 1725. Jean-Baptiste GUILLEMOT dit Lalande remarried to Charlotte Marchand on 20 Fev 1734 in Montreal, the church record mentioning he was the wodower of CatherineOUABENAQUIQUOY.
I also note that Jacques GUILLEMOT dit Lalande, a brother of Jean-Baptiste above, was also married in Kaskaskia, in about 1714, to Marie TITIO or TEKTIO, also an indian woman.
Finally, I have discovered that Marie-Chrétienne GUILLEMOT dit Lalande, a sister of Jean-Baptiste and Jacques, above, born in Montreal on Sept 28, 1695, was "hired to work in the West" on Aug 30, 1710, i.e. at age 15. Did she follow or accompany het two brothers to Kaskaskia ? But she obviously came back to Canada, where she was married in 1715 and gave birth to children in Trois-Rivières bwtween 1720 and 1724.
Might she have been accompanied by a sister of hers, that 'mysterious' Mathurine whose birth we cannot find? And who would have met AUFRERE out "West" ?
I even wondered whether "Mathurine" might not have been born in Kaskaskia, but if she was already married to AUFRERE in about 1723-24 (but we have no proof that she who shows as AUFRERE's wife with a 3-year old child in New Oerleans in 1727 was indeed Mathurinbe, have we?) we must rule out the possibility that she might have been a child of either François or Jean-Baptiste, above ....., unless born before those two marrried.