Looking at the list the OP made, what you are saying is that Mary can make those promises herself without consulting the King - that she is completely within her rights in making those promises?
Remember that Mary, being in heaven, is perfect, since nothing imperfect can enter. Likewise all the heavenly Saints.
Being that they are perfect, then their minds, hearts and wills are perfectly united to God. So there is not the slightest chance of their thinking, saying or doing anything that is not perfectly in accord with God’s will.
And there is not the slightest chance that they will in any way misuse any power that He chooses to give them. So if Mary IS allowed freedom to act independently (and why would she not be, those in heaven are neither slaves nor robots), she will in no way do other than what God wills to be done. This includes her intercessions as well as any other favors it may be in her power to bestow.
That’s confusing. IF the church, as you said, having been guided by the HS, had found this private revelation to be authentic, why could it still be possible that this is something that is not really true because it may have come from a troubled mind?
This is an instance of humility. The Church does not pretend to know absolutely everything there is to know about God, for such complete knowledge is impossible for even the most Spirit-guided person or group of people. As St Paul says, for the time being we must see through a glass darkly.
However, there are things that it does know beyond a shadow of a doubt. And these take precedence over any private vision or revelation.
As far as any private revelation goes, all the Church can do is scrutinise them and then declare that the messages of some of these private revelations in no way contradict the revealed and certain aspects of our faith. And that being the case, the faithful are permitted, without being required, to believe them.
Sometimes after examination it does the opposite - it finds that there are aspects of some so-called revelations that do contradict revealed faith, and it warns the faithful against those.
In both cases its pronouncements about the revelations do not have the force of eternal and binding-for-all-times dogma. Although in obedience we should comply by avoiding that of which our spiritual fathers disapprove.
In one case, that of St Faustina Kowalska, a grave mistake was made due (I think) to poor translation of her writings. The Church hierarchy was against them for a time but then when John Paul II, a native of Faustina’s own country, became Pope matters were rectified.