There are different things evaluated here.
The thread title asks about authenticity of the visions. I don’t think that matters to anyone, exception of St Faustina. If her ideas came from a vision of Christ, or from a prayerful summarizing of Scripture, Tradition, and spiritual direction, they are the same ideas. The Church isn’t basing approval on her visions but on the diary, and her overall holiness.
The Chaplet of Divine Mercy is a Private Devotion. Like all approved private Devotions it must draw its strength from, and bring us back to, the Mass.
The Diary is Private Revelation. It draws its credibility from, and should lead us back to, Public Revelation. Faustina’s Diary contains many spiritual insights besides remarks attributed to Jesus and Mary. It is kind of a Midway point or mixture between something like Fatima, and something like reading St Teresa of Liseaux, etc.