Clearly had Grouchy used his brain and followed the sound of canon fire he would have prevented or delayed the Prussians. Instead of following the break he merely stuck to the original plan.
One Le Hay-Saint farm fell it was over for the British as the centre of their ranks was broken... had Grouchy's men appeared on Napoleon's right flank, instead of Blucher (neigh) then it would have been the end for the dastardly Wellington. Instead of the peloton rejoining it was split further by the breakaway Prussians.
Napoleon spent most of the battle away from the field, allowing Ney to direct the action... he was the bravest of the brave, but also the stupidest of the stupid... cavalry charges unsupported by infantry? what was he thinking? the men were out front and vulnerable, no one rode to help them.