Our Lady gave us the Rosary as a weapon and as one of the the most powerfull sources of grace that there is. The deception does not lie with the power of the Rosary to preserve the faith at all. It has nothing to do with the Rosary. The deception lies in the power of the "Eucharistic liturgy" to preserve the faith. That's the lie. The "Eucharistic liturgy" has for its fruits, the loss of faith, not its preservation.
What "half truths" of this "Fr. Gruner" are you talking about? Do you have one or two real good examples?
Aside from my reply above, this answer could easily take up more space than I am able to write, but Fr. Gruner's preaching that the Consecration of Russia will result in Russia's conversion is a half truth. Without the Holy Sacrifice of the Mass, where will their conversion lead them - and us all too -........perhaps the Santa Mass where communion is a cookie instead of a host and is distributed by elves as is common place in todays happy Church?
The 3rd Secret was given in 1917, when there was no such thing as a Novus Ordo. If Our Lady refers to the Eucharistic liturgy in it, she would have to be talking about the Traditional Latin Mass
Eucharistic liturgy = novus ordo service = responsible for the unprecedented loss of faith, successfully (so far) instituted to replace the Holy Sacrifice of the Mass - or as you called it, the Traditional Latin Mass. Do not confuse the "Eucharistic liturgy" with the Holy Sacrifice of the Mass, I assure you that Our Lady would not confuse them as they are complete opposites, promulgated by the enemy by design.
Our Lady could have said: the Rosary is, after the Holy Sacrifice of the Mass, the prayer most apt for preserving faith in souls, but She could not have said: after the holy Eucharistic liturgy as the two are opposed to each other.
I suggest that the "Real" third secret warns of the devastation that the Church (and the world) would suffer at the hands of Russia when the Holy Sacrifice of the Mass was replaced with the Novus Ordo and its "Eucharistic liturgy".
I suggest you are making stuff up to coincide with your campaign against the Mass of Paul VI.
The Novus Ordo is the Holy Sacrifice of the Mass, no matter how much you say it's not.