Let me clarify - the question of who Napoleon should have chosen before the campaign began as wing commanders is what I think is an incorrect question as those roles did not exist. I would agree that as the campaign unfolded, the roles that Ney and Grouchy ended up having are of paramount importance. You are correct.
The left wing was 2 Corps because Napoleon expected it and his reserves to be the main force on a drive towards Brussels the night of June 16th. What Napoleon "knew" the night of June 15th was 1) no british in sight, 2) Quatre Bras occupied, 3) 1 mauled Prussian corps on the retreat and 4) a critical mass of the Anglo-Dutch and Prussian forces too far away to be expected to give battle.
Even though Napoleon's presumptions were absolutely wrong on the morning of the 16th (Anglo-Dutch retreating, only 1 Prussian Corps able to reach Sombreffe) his preparations and planning were such that he could account for any situation. (as will be demonstrated, Napoleon was actively deceived on June 15th/16th, and thus he did not have an accurate picture of reality. This will be shown with the correspondence during the campaign, as well as the memoirs. Napoleon went to his death without ever learning the true disposition of his left wing on June 15th, and was given a false explanation on June 16th.)
Thus, when it was discovered that the Prussians had somehow concentrated more quickly (due to treason, the concentration began the night of the 14th) this offered Napoleon an opportunity to destroy one of the allied armies.
Hence, I would argue that the Allied plans of where to concentrate in the event of an advance via Charleroi completely underestimated Napoleon, and were risky. Those that argue that the allies had somehow acquired Napoleon's pixie dust and understood how the new age of war was while Napoleon was caught with some antiquated view completely ignore that only (treason)(unbelievable incompetence)(both) spared a decisive defeat on the 16th.
Having said that, one cannot deny that the Allies created an opportunity, albeit it was dangerous(or let's say bold!), by having an aggressive response to the invasion, and then performed tenaciously.
As a tangent, this source offers a lot of different views on June 15th/16th regarding Ney, including some unique ones drawn from the stories the family he stayed with passed down:
Starting on page 321.
One theory that this discusses that I just heard recently on Napoleon1er.org, and this may support... is that Ney was hitting the bottle during the campaign and that explains why he was no where to be seen in the early morning hours of June 16th! Yet, there is correspondence and memoirs that refute this - so very possible the family's stories have grown with time, but when time permits, I plan to revisit this concept.
Even if Ney was ordered to take QB on June 15th, based on the tone Napoleon set himself, there was no urgency for the left wing on the 16th, and only hindsight (or strong desire)(or frustration by fans of Napoleon) makes taking QB a strategic necessity.