You did find a lot of info about this William.Last Sunday, I searched Drouin's database for William and Guillaume but did not find anything meaningful with the exception of a William born in St-Chrysostome.
Just a few clarifications:
William and Deseline also had a son name WIlliam, born in 1872 I believe; he married Marie Lessard dit Cloutier in 1892.
Mother of Desiline: her marriage record shows Martine vs Mathilde or Mathilda (Ref. BMS2000).COuld be a transcription error because Martine in that timeframe was not a common given name.
William's second wife:Given that she was born in Québec, her surname is probably Blais vs Balis.
Place of birth info for William at the time of his second married: should be St-Vallier (as you have found his birth record) in lieu of Ste-Henedine (not St-Andeline).
William's Parents: Hubert Gagnon and Celanire (vs Cesarie) Garant.
From the marriage register of Waterville, I have the marriage data for: William C Adele (Addie) Octave (David) Henry and J. Philias
Other children of Hubert Gagnon and Celanire Garand (Garant): of those you have listed, I have not find marriage records for any of them with BMS2000.I will make another check with another database.
M2: for Hubert Gagnon to Flavie Roy took place on 27 Jul 1858 at St-Anselme (Dorchester Co.).
Given the names of William's parents that you have found, one is then in a position to complete Benedict's family tree up to the arrival of his ancestor in New France in the mid-1600. There were 3 Gagnon brothers and a cousin by the same surname.
It is obvious that Benedict did not check the block: « Notify me whenever someone replies to my messages ».It is unfortunate in your case mainly and in mine to try to help and not getting acknowledgment of one's input.Hopefully, he will check this forum soon.