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In Scripture and Tradition, it is plain that the Lord Died for us and Sacrificed His Life on the Cross for us at 3:00 P.M on Friday. This was also the Hour when His Heart was pierced by the Holy Lance of St. Longinus, and Blood and Water flowed forth from that pierced Heart, as St. John the Apostle records for us in Jn 19:34-35. See also 1 Jn 5:6. The Hour of 3:00 P.M is called the Ninth Hour (from 6:00 A.M. in the morning) or Nones in Scripture and the Divine Office. Thus, "In addition to Morning and Evening Prayer to accompany the sacrifices, there was prayer at the Third, Sixth and Ninth Hours of the day.[5] https://en.wikipedia.org/wiki/Nones_(liturgy)

Early Church

The Apostles continued to frequent the Temple at the customary hours of prayer (Acts 3:1): "Now Peter and John went up into the temple at the ninth hour of prayer."[5] At an early date mystical reasons for the division of the day were sought. St. Cyprian sees in the hours of Terce, Sext and Nones, which come after a lapse of three hours, an allusion to the Trinity. He adds that these hours already consecrated to prayer under the Old Dispensation have been sanctified in the New Testament by great mysteries—Terce by the descent of the Holy Spirit on the Apostles; Sext by the prayers of St. Peter, the reception of the Gentiles into the Church, or yet again by the crucifixion of Christ; Nones by the death of Christ.[6] St. Basil merely recalls that it was at the ninth hour that the Apostles Peter and John were wont to go to the Temple to pray.[7] St. John Cassian, who adopts the Cyprian interpretation for Terce and Sext, sees in the Hour of Nones the descent of Christ into hell.[8] But, as a rule, it is the death of Christ that is commemorated at the Hour of Nones."

So this has a very long history, yet is ignored by many Catholics, who choose to remain ignorant of the time when the floodgates of Mercy opened up for the whole world, which Jesus Our Lord means us to commemorate and remember every day, with Grace if we do.

These Promises are from God and were given to St. Maria Faustina for our spiritual benefit. It's up to us to make use of or ignore them.

There is no question of any false mercy here, because the primary area God promises Mercy is in the Confessional after Repentance. What Jesus Promises in His Divine Mercy Revelation, just like in His Sacred Heart Revelation, is to give us His Mercy also through Prayer.

"Hence, the Lord said, "At three o’clock, implore My mercy, especially for sinners; and, if only for a brief moment, immerse yourself in My Passion, particularly in My abandonment at the moment of agony. This is the hour of great mercy. In this hour, I will refuse nothing to the soul that makes a request of Me in virtue of My Passion (Diary, 1320).

As often as you hear the clock strike the third hour, immerse yourself completely in My mercy, adoring and glorifying it; invoke its omnipotence for the whole world, and particularly for poor sinners; for at that moment mercy was opened wide for every soul. In this hour you can obtain everything for yourself and for others for the asking; it was the hour of grace for the whole world — mercy triumphed over justice. (1572) [cf. Jam 2:13 for a similar passage - Xavier]

My daughter, try your best to make the Stations of the Cross in this hour, provided that your duties permit it; and if you are not able to make the Stations of the Cross, then at least step into the chapel for a moment and adore, in the Most Blessed Sacrament, My Heart, which is full of mercy; and should you be unable to step into the chapel, immerse yourself in prayer there where you happen to be, if only for a very brief instant. (1572)" See also: https://aleteia.org/2018/01/23/what-is-the-hour-of-great-mercy/

We should make use of this, and never let the Ninth Hour, or 3:00 P.M. in the day pass, without asking great graces from God for ourselves, and our loved ones; graces of conversion and of salvation for sinners about to die, grace of sanctification and of perseverance for the just; graces for us and all our families and friends to embrace the Faith or remain in and increase in Charity and holiness every day; and any and every grace we desire for ourselves or for the whole world, on the strength of Christ's bitter Passion.

“On Good Friday. At three o’clock, I saw the Lord Jesus, crucified, who looked at me and said, I THIRST. Then I saw two rays issue from His side, just as they appear in the image. I then felt in my soul the desire to save souls and empty myself for the sake of poor sinners. I offered myself, together with the dying Jesus, to the Eternal Father, for the salvation of the whole world. With Jesus, through Jesus and in Jesus is my communion with You, Eternal Father.” (Diary, 648)

“Encourage souls to say the Chaplet which I have given you … Whoever will recite it will receive great mercy at the hour of death … When they say this chaplet in the presence of the dying, I will stand between my Father and the dying person, not as the Just Judge but as the Merciful Savior … Priests will recommend it to sinners as their last hope of salvation. Even if there were a sinner most hardened, if he were to recite this chaplet only once, he would receive grace from my infinite mercy. I desire to grant unimaginable graces to those souls who trust in My mercy … Through the Chaplet you will obtain everything, if what you ask for is compatible with My will.”

Why do you have to rope Faustina into this?  It's clear enough from Sacred Scripture that Our Lord died at or around 3PM.

You're just a private revelation junkie, aren't you?

Suit yourself. You will miss out on the graces the Lord gives at the Hour then. Something like 25 million Catholics before 1953 practiced Divine Mercy. You'd have to be quite closed to grace to think it was the devil who wanted to remind us to meditate on Christ's Passion, informing us of the hour of His Death correctly, which millions of Catholics still do not know and have not troubled to find out, thereby losing out on countless graces. Jesus gave a simple way even for those not learned in Scripture and Tradition to find out. To you, it was a demon who said to meditate on the Stations of the Cross in that hour, on Eucharistic Adoration, and other exercises to save souls. Either that, or St. Faustina supposedly invented it herself, or some other contorted explanation like that.  

It is love for souls and zeal for their salvation that really counts with God, as the Saints say: "in the evening of life, we will all be judged by our love" (St. John of the Cross). And St. Faustina had this abundantly, and very many were saved by her prayers in her own lifetime. Almost every Saint faces opposition at first, that proves less than nothing. Even St. Padre Pio faced opposition from very many. But God ultimately vindicates His own. Always.

The Value of our Personal Suffering

A special role in entreating God's mercy for the world is played by suffering. "There is but one price at which souls are bought”, Jesus told St. Faustina, “and that is suffering united to My suffering on the cross" (Diary 324).

"Every conversion of a sinful soul demands sacrifice" (Diary 961). The Lord Jesus told St. Faustina: "I have need of your sufferings to rescue souls” (Diary 1612).”Help Me, My daughter to save souls. Join your sufferings to My Passion and offer them to the Heavenly Father for sinners" (Diary 1032)." https://www.gla.ac.uk/myglasgow/catholicchaplaincy/divinemercy/chaplet/savingasoulmaydependonyou/

On the last day, the calumniators of the Saints will risk being placed among the enemies of God. Go read, if you wish to learn the Truth, what the Doctors say about those who knowingly and willfully deny the Church's Infallibility in Canonizing Saints, and then slander the Saints. I suppose, like you said St. Robert was "emoting", St. Alphonsus was one of those "heretics (objectively speaking)" because they disagreed with you on the Church Teaching of Baptism of Desire, you'll think they were wrong on this one also. An infallible judge of these things you most certainly are not, that's really the mildest way to put it.

"O Jesus, eternal Truth, our Life, I call upon You and I beg Your mercy for poor sinners. O sweetest Heart of my Lord, full of pity and unfathomable mercy, I plead with You for poor sinners. O Most Sacred Heart, Fount of Mercy from which gush forth rays of inconceivable graces upon the entire human race, I beg of You light for poor sinners. O Jesus, be mindful of Your own bitter Passion and do not permit the loss of souls redeemed at so dear a price of Your most precious Blood. O Jesus, when I consider the great price of Your Blood, I rejoice at its immensity, for one drop alone would have been enough for the salvation of all sinners. Although sin is an abyss of wickedness and ingratitude, the price paid for us can never be equalled. Therefore, let every soul trust in the Passion of the Lord, and place its hope in His mercy." https://www.thedivinemercy.org/news/Saint-Faustina-and-the-Sacred-Heart-5872

Fr. Ripperger is just one Traditional Priest and Exorcist who knows by experience how powerful the Divine Mercy is, but you know better than Father R., I'm sure. Fr. Isaac Mary Relyea is another and there are many others.