Fouché gained control of a government that lasted how long? Do you think Fouché considered the end result a success?
Underestimating or misjudging the politics is exactly the point.
At the time moves were made, a Bourbon restoration was doubted by some, believed impossible by some.
Soult claimed in his justification that he only served Napoleon to protect the country from invaders, how could a Marshal of France not draw is sword during such a time! Yet precisely when he was needed most - he was off to Paris... just what was Soult up to!?
It seems that the only explanation you will except is one with a great master plan, and one that succeeds. Soult was a liar in 1815, and combined with his silence that extends beyond the grave, it is very difficult to pin his motivations down. But what is certain is that there is not a very good explanation for his combination of changing orders and keeping intelligence from Napoleon - and then somehow these events are hidden for almost 100 years. If he did so willfully, he was a traitor.
My bar for calling Soult a traitor is pretty low. In a military situation, if one willfully disobeys a superior officer during time of war, they may be punished by the death penalty. There is no simple label for that person, so for the purposes of my work, I simply grouped all during 1815 that were willfully not doing their best as traitors. Hence, the garrison guy that thinks the food sucks, the hours are rough, and the uniform itchy who took his 20 Francs and crossed over is a traitor. The paid spy from Ghent who was a committed Royalist and loved King and Country who smuggled War Department papers out of Paris is a traitor. Few weeks later he might be a Super Patriot - but he was a traitor to Imperial France.
And one of my theses is that Napoleon and his efforts were massively betrayed in 1815 - to a far greater extent than typically known.
And pending some reasonable explanation on Soult's actions, he was one of the traitors, but that does not mean he was in Fouché's league as a schemer - but one can note that in the long run Soult was handsomely rewarded by a French monarchy.