One of the often asked questions is, "Who should have Napoleon chosen for his wing commanders other than Ney and Grouchy."
Yet Grouchy was in charge of the Reserve Cavalry, and as you point out, Ney did not have a role in the Armée du Nord until he finally got Napoleon's attention in the afternoon of June 15th. (some controversy about when, and interesting stories of how Ney showed up the day before, had to buy horses from Mortier, Soult's only friend in the army, who came down with a very convenient case of Gout, and finally caught up to Napoleon around Charleroi... I think Pierre de Wit actually has a theory on the exact place.)
Napoleon did intend 3 columns with his June 10th order (and of course, in a few hours, things get really interesting....) but they converged at a common "point". So one cannot even say with certainty that the 2 wings and a reserve was his plan north of the Sambre.
When did Napoleon decide on Wings and a reserve? Was it always in his mind? Or was it spontaneous on the morning of the 16th?
Did Ney's arrive contribute to the thought?
All interesting questions, and I think the detail I'll add over next few days will give some material to chew on. Napoleon has been criticized for giving commands to Ney and Grouchy... I used to really think it was stupid... but that was based on all the conventional histories that are weak.
When one replays the campaign slowly, one sees that Napoleon expected Ney to hold tight, and Grouchy, though commander of the Right wing, was superseded when Napoleon was present. Grouchy had a few hours command before Napoleon took charge on the 16th, and we all remember what Grouchy did during the battle of Ligny, right? I think its fair to say his son did more...
So in Napoleon's mind, he has these two guys in charge of wings - but he is going to take lead first on the right, and then once he has pushed this single Prussian corps past Gembloux, he leaves Grouchy and some force to protect the rear while he takes lead on the left and force marches to Brussels that night. Based on what Napoleon knew on the morning of June 16th, Ney and Grouchy were NOT going to be exercising significant independent commands with great responsibility, but because Napoleon had a mission in front of him, it seems he did feel it important to put the Marshals formally in charge.
But the entire concept of this big wing commander choice is very much overstated. Grouchy was in charge of the reserve cavalry, and Ney was given an invitation that according to anecdotes, Napoleon didn't even expect him to accept.