Even if Ney attacked earlier and took the crossroads, wouldn't he be so strung out and exposed that Wellington can come down on him like a ton of bricks with the rest of his forces. If like OTL D'Erlon's corps was paralyzed with contradicting issued commands, can Ney's wing hold or more importantly once Ney realizes Wellington's got numbers and is coming en masse, will he even TRY to hold? By this point i'm sure riders have been sent to find out where the fuck D'Erlon's corp is, and word has gotten back that he aint gonna get there before Wellington's merry men.
Thus Ney likely gets the hell out of dodge after scoring more kills than OTL, but will it be enough to swing the next coming battle. Perhaps if Ney really wrecked some shit, and Wellington hear's how the Prussians got stomped, he might just decide to turn tail to the sea, preserve his army to threaten Napoleon when the rest of the coalition marches to the front.
Regardless, Nappy really wasnt on his game by that point of time and the Coalition is straight HUGE. Woulda been interesting to see how long he can holdout if he were healthy and had all his best marshals/generals with him.
If Davout was at Quatre-Bras he'd prolly carry the crossroads, made damn sure D'Erlon was coming to reinforce him and shatter Wellington's troops as they try to swarm in piecemeal to take on his exposed wing or take time to form up giving D'Erlon time to reinforce, further showing why he was prolly the best commander of the Napoleonic Era. Then of course Nappy would just claim that temporarily repulsing the Prussians was waaaay more important than shattering of Wellington's reputation and forcing the British off the continent for awhile
