In Discord:Not One of A Kind, putting aside that Discord is the personification of Chaos and Fluttercruel is more or less his daughter, it makes perfect sense for his healing of her to be a little off. It's the first genuinely nice thinghe's ever done in his entire existence: he's not used to doing it.
It would be rather silly for Discord to be able to use the elements of harmony, since they are what locked him in stone the first time. However, consider. Discord was meant to be in concerto with his Sisters from the beginning of time before he fell. So now that he's had a Heel–Face Turn, he's now worthy to have elements of harmony in concerto. Kinda a final show that he has been redeemed for the alicorns.
