Late on the 14th, early on the 15th... as with this detail, was hard for me to pinpoint.
There are cartoon caricatures of Mortier suffering while the Army advances... I have friends with Gout, and it is painful... some might compare it to getting an amputation! Oh wait, the Colonel who had that happened mounted right back up and fought the next day at Waterloo... I guess it becomes a subject of motivation...
Of course, Mortier dropping off is not evidence - but it is a coincidence that Soult's only friend in the army (and great friend of Louis-Philippe) would bow out while so many guard Generals would be wounded or killed.
Likewise it is a coincidence that the most openly anti-Soult officer, Vandamme, would be the orders that were lost... (too funny as well that on the orders of the 15th, for the first time in all the orders of the day, it is Vandamme that is listed first! Hmmm, was he on Soult's mind? Or was Napoleon just dictating starting from the center since that is where he was going to be.)
Likewise it is a coincidence that Soult's order change just happened to put the one general that defected nearest the Prussians.... couldn't have planned it better!
With Fouché and Bourmont both communicating directly and with Ghent, is it possible Soult was in that chain, or that he learned something useful in Lille from one of his many former associates that were in Ghent the week before the campaign started?
All this is just noise! Better to stick with the black and white evidence, it is sufficient!