I don't agree that the wing commander topic is overstated, it is understated, now in case those two Marshals found themselves in command of an "Army" instead of the usual corps, using an independent staff etc.
This would need quite a preperation to do all this and both had lack of experience.
Whereas the Allies had two trusted Army commanders who knew what they were doing. The collaboration Wellington - Blücher was severly criticised - especially at the 16th of June, but I cannot find that big fault on that - they acted in concert and one or the other pinned French units which might have been vital to accomplish a bigger success than accomplished at Ligny or Quatre Bras.
In case Ney - as I understand it as well, should just brush away some battalions and march almost unopposed to Brussels, why give him such a big army?