It was approved by JP2 shortly after he became pope. From what I had read, he had been trying to get his predecessors to approve it for years prior to acceding to the papacy.
Considering that private revelations are non-binding. We can agree to disagree about its authenticity.
Byzantine Christianity is 100 percent opposed to "private revelations" of any kind. Actually, the Roman Church's acceptance of it is an ecumenical obstacle with the Orthodox.
Since Sister Faustina has been canonized, meaning that her claims have been approved by the Church, I will believe them. She would not be St. Faustina if they were heretical as you claim.
As a new Catholic, I am puzzled why you are making such an attack on a Latin Rite feast day. I don't suppose you would appreciate it if we attacked your feast days.