Well Personally (as british from what I have read from many sources Napoleon messed up tactically big time. Wellington took a beating at quatre Bras but held the army together at Waterloo. People need to remember many of the british battalions were not his vets from Spain (had he his Veteran battalions quatre bra would have been a victory.). To me the battle was turning as the Prussians arrived and is rightfully an allied victory to say it was either Prussian or british alone is ludacris.
Had a better commander than Grouchy been in command of the detached corps Blucher would never have made it to waterloo because someone like Davout would have cut his strung out army to pieces.
What has played in my head is would that corps reach waterloo in time to stop the french forces wavering? I don't think it would have and Wellington would have been able to retreat to the coast.