If I was Soult and had carefully planned to ruin the campaign in the interests of my future career I think I would have stayed with the army, taken charge (quite properly), kept it out of harm's way (unless a few trouble-makers could be sacrificed) and then, when the time was ripe, declared that the interests of France lay in declaring for the King. This was the decision Davout made in the end. I would not have thrown all my trouble away by acting as he did which gained him nothing. On your showing he had Bourmont to speak for him at Ghent so negotiations should have been quite easy but I have seen no indication from the Royalist side that they had any hopes of him.
This is where your argument still has problems I think.