It is not known for certain, but it is very likely the case. That is what most fans seem to think and it has been hinted at in a tweet by one of the writers.
It is not known what these flashes of rainbow in the pony's eyes is supposed to represent, but this will likely be revealed in the Season Finale of Season 4 of My Little Pony: Friendship is Magic.
amity, empathy, peace, unity, friendship
Most likely
No, that box transformed into castle
It depends on the context. If you mean: musical harmony-melodiousness, tunefulness harmony of a structure-balance, symmetry, compatibility living together in harmony-peace, friendship, accord
The My Little Pony Friendship is Magic Rainbow Power toys are connected to the box on the Root of the Tree of Harmony show.
S2 E1 "Return of Harmony"
Hope Friendship Harmony Teamwork
Harmony Lane - 1935 is rated/received certificates of: USA:Approved
Harmony - 2005 is rated/received certificates of: USA:PG-13 (DVD rating)
Too Much Harmony - 1933 is rated/received certificates of: USA:Approved
Perfect Harmony - 1991 TV is rated/received certificates of: Australia:PG