What you say is true, but Nestorius also was tried at the Ecumenical Council of Ephesus, although the Archbishop of Old Rome had already condemned him.
St. Meletius did not preside at the council of Ephesus but died 50 years earlier.
I think Montalban is perhaps conflating two events.
The first is the St. Meletius presided over the First Council of Constantinople, despite being out of communion with Rome.
The second is that Dioscoros (not Nestorius) was not immediately denied a seat at Chalcedon, despite the protestations of Pope Leo's legates, who had instructions from him not to allow Dioscoros to take a seat (because Pope Leo already considered Dioscoros to have been guilty of misconduct and had excommunicated him). The council itself only denied Dioscoros a seat once charges were brought against him not for the reason that Pope Leo had excommunicated him, but because one cannot judge his own case.
I very much doubt that Nestorius was received at Ephesus with all due honor, but then perhaps I am remembering wrong. I know for sure, however, that Dioscoros was treated with due honor at Chalcedon, before he was found guilty.
The emperor had called for bishops to assemble in the city of Ephesus. The emperor sent as his representative Count Candidian (Bury, J. B., (1958), History of the later Roman Empire : from the death of Theodosius I to the death of Justinian (A.D. 395 to A.D. 565) (Volume 1) ,(Dover Publications; NY), p353)
Two groups of bishops gathered in the city. One group of bishops formed around Cyril of Alexandria. The other group centered around Nestorius
(Hill, B. R., (2004) Jesus, the Christ: Contemporary Perspectives, (Twenty-Third Publications), p230.
Guy, L., (2004) Introducing Early Christianity: A Topical Survey of Its Life, Beliefs and Practices, (InterVarsity Press) p291.)
Both groups had duly assembled in Ephesus as commanded by the emperor. But which was the official council? Candidian was unable to control proceedings; to bring the two groups together
Runciman, S., (1977), The Byzantine Theocracy, (Cambridge University Press), p37
Cyril had canonical grounds for opening the first session... "Nonetheless he must have been acutely aware that he could claim no legal status for his synod under imperial law until the official reading of the Emperor’s Sacra had taken place.”
McGuckin, J, (2004), Saint Cyril of Alexandria and the Christological Controversy, (St Vladimir’s Seminary Press; Crestwood, NY), p78.
Cyril, in order to constitute a legal council needed to have the backing of the emperor, or his representative. However Candidian was a supporter of Nestorius.
Chrystal, J., (1985) Authoritative Christianity (Volume One) (James Chrystal Publisher; Jersey City, NJ), p50.
Candidian went to the group supporting Cyril and demanded that they reconvene with Nestorius’ group. He said he had come with the Sacra and had no time to stand around and wait. Cyril asked him what did the Sacra say. Candidian read it out before Cyril’s group. He then realised that he had now formally given the go-ahead for Cyril’s group to begin as the council; because they were duly assembled as according to the wishes of the Emperor and the Sacra had been read before them. They thus convened the council, and effectively Nestorius’ group were left out in the cold.
“When Candidian finished reading the Sacra he surely realised the full extent of his mistake. The Bishops acclaimed long life to the Emperor in demonstrative professions of loyalty, but now with the text officially declaimed in the symbolic presence of the whole Episcopal gathering the Synod of Ephesus was in formal session, legally as well as canonically sanctioned.”
McGuckin, J, (2004), Saint Cyril of Alexandria and the Christological Controversy, (St Vladimir’s Seminary Press; Crestwood, NY), p79.
The group that had been duly assembled, with the Sacra of the Emperor read before it made it a legal council in the eyes of the civil authorities. By having the Sacra read out it served to legally validate the subsequent Acta
McGuckin, J, (2004), Saint Cyril of Alexandria and the Christological Controversy, (St Vladimir’s Seminary Press; Crestwood, NY), p81.
The council was only legal when the emperor’s representative had opened it, not as Catholics would imagine; solely because the Pope commanded it, or because the Pope's representative (as they imagine Cyril to be) was there.
The important facts therefore are:
Nestorius had already been condemned by the pope but attended a council for the council to decide the matter. He had support, such as John of Antioch.
The matter was settled by a council judging for itself.
Nestorius (potentially) could have chaired the council (although the numbers of bishops in his circle were fewer then those around Cyril).
I am writing a book about this.