Do not Forget the Popes
It is worth examining this issue for a moment. If one were to consider the authoritative weight of all the statements made by curial offices and clergy, those with the greatest authority ultimately resides in the Chair of Peter; the Holy Roman Pontiff.  

First Vatican Council, Session IV, Chp. IV, p.IX "So, then, if anyone says that the Roman Pontiff has merely an office of supervision and guidance, and not the full and supreme power of jurisdiction over the whole Church, and this not only in matters of faith and morals, but also in those which concern the discipline and government of the Church dispersed throughout the whole world; or that he has only the principal part, but not the absolute fullness, of this supreme power; or that this power of his is not ordinary and immediate both over all and each of the Churches and over all and each of the pastors and faithful: let him be anathema." Catechism of the Catholic Church, 883 "The college or body of bishops has no authority unless united with the Roman Pontiff, Peter's successor, as its head."

Therefore, let us now examine exclusively the actions of the popes on this matter, apart from the Holy Office;

[1948] Pope Pius XII: Granted permission to publish the writing "as is"
[1959] Pope John XXIII: Signed the decree placing the Poem on the Index
[1966] Pope Paul VI: Suppresses the Index
[1966] Pope Paul VI: Abrogated Canon 1385, which was the legal principle for the inclusion on the Index
[1974] Pope Paul VI: Authorizes a letter of appreciation of a commentary on the Poem
[1992] Pope John Paul II: Beatifies an outspoken supporter of Maria Valtorta and Scripture Scholar
[-------] Pope Benedict XVI: pending...

Thus, we observe three popes, whether directly or indirectly, supporting the writings of Maria Valtorta. The action of the one pope who signed the decree placing the Poem on the Index, Pope John XXIII, provoked an immediate response by his successor, Pope Paul VI just a few years later (as was the legal principle for the condemnation). And we know from canon law that only a pope can abrogate, modify, or contradict the actions of a previous pope.

These actions, while they do not constitute formal approval, nonetheless are reflective of the mind of Christ's shepards on earth, who have "full, supreme, and universal power over the whole Church, a power which he can always excersice unhindered." Indeed, the divine protection granted to the office of the chair of Peter is what distinguishes him as the visible head of Christ’s body on earth. And as we have seen with mystics such as Saint Padre Pio and Saint Faustina (both of whom were condemned by the Holy Office), it was ultimately a pope who positively discerned their authentic lives of heroic virtue.

It is also worthy to note that, among the thousands of apparitions throughout history, very few have received the attention of a pope. ² In fact, most alleged apparitions are not reviewed beyond the jurisdiction of the local ordinary—if they are ever reviewed at all. Fewer still are reviewed by the Congregation for the Doctrine of the Faith. Thus, for an alleged apparition to be given direct recognition by a pope is not insignificant. All things considered, one may conclude that to grant unequivocal authority to the Holy Office’s temporary inclusion on a suppressed Index, would not be in keeping faithful to the facts of history in light of the teachings of the Church.

Related Article: A Critical Analysis of the Letter of Condemnation by the Holy Office

NOTES
1. Research by David J. Webster M.Div. 2004 [link] [source]
2. Maria Valtorta and Her Epic Narrative [link]