D'Artagnan did really exist. His name was Charles de Batz and was called D'Artagnan when he arrived in Paris probably because he was coming from the south-west of France (where the movie was partly shot) and where is the little city of Artagnan.
Just before the Musketeers charge the castle for the final fight scene, d'Artagnan is seen in his normal clothes, then in his father's tunic, then once again without the tunic.
During the final ladder fight scene, D'Artagnan stabs Febre through the ladder after sliding all the way to the bottom of the ladder, but in the next shot as D'Artagnan walks away Febre's corpse is 8 feet farther up the ladder.