Hi.I recently determined that Louisa Derouin was my 3rd GGM. At least it looks like that name on the record.She was married to James Watz.Their daughter was Marie Louisa Watz (b:1832 in St Charles, MO).Marie Louisa Watz married Jean Baptiste Piant (Payan) of Florissant, MO on 11 April 1850 at the St Louis Cathedral.Does any of this fit in with your research?
You mentioned Mary Ann Guitard.Could that be Marie Anne Guitard (b:1745) who was the daughter of Jean dit Legrandeur Guitard and Marie Anne Meloche?
I also ran across a circuit court record that Francois Derouin was charged with assault on Macky Wherry, sheriff in St Charles, MO in October 1807.No other info on that site regarding the case. Regards Mike Burke