Of course Diaval is the one who knows human social norms like small talk and non-creepy smiling, he spent sixteen years spying on humans!
Not to mention that even before that, he likely spent a fair deal of time exposed to humans as a raven (Corvids being notably intelligent avians)
Early in the movie, the Queen is aiming a crossbow around the room and her general informs her that it's armed. Then everyone else walks in on her and she tosses the crossbow to the general, when it suddenly fires and hits a statue that sorta resembles Maleficent. This is foreshadowing to multiple points:
The Queen actually poisoned her husband and would blame it on someone else.
Even though the Queen set up the trap, the general was the one who carried it out.
Queen never got to kill a fairy, but the general managed to shoot some of them.
Ingrith being turned into a goat at the end is also a fitting transformation in that after using Maleficent (and fairy species) as scapegoats during her reign for years, Maleficent would turn her into a real goat.
The tagline "Evil never looked so good" could be assumed to be referring to Maleficent, which is odd since she is actually an example of Dark Is Not Evil. However, you than realize that it was more likely referring to Queen Ingrith.
