V—Although in all these meetings and conferences any communication whatsoever in worship must be avoided, yet the recitation in common of the Lord's Prayer or of some prayer approved by the Catholic Church, is not forbidden for opening or closing the said meetings.

Quote from: Pelele

This decree is regarded as a condemnation of false ecuмenism and as a reiteration of Mortalium Animos, and indeed it is, yet how is it that in #V it still allows for common prayer with heretics?

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V—Although in all these meetings and conferences any communication whatsoever in worship must be avoided, yet the recitation in common of the Lord's Prayer or of some prayer approved by the Catholic Church, is not forbidden for opening or closing the said meetings.

How is this possible?

Reread Mortalium Animos.

#9: Everyone knows that John himself, the Apostle of love, who seems to reveal in his Gospel the secrets of the Sacred Heart of Jesus, and who never ceased to impress on the memories of his followers the new commandment "Love one another," altogether forbade any intercourse with those who professed a mutilated and corrupt version of Christ's teaching: "If any man come to you and bring not this doctrine, receive him not into the house nor say to him: God speed you."

I've read this decree, and to understand why it is unproblematic, please read Sess. XIII Chap. VIII on the safe-conduct granted to the Protestants and Sess. XV in full of the Council of Trent on the same to see how the Fathers there invited in a friendly tone the Protestants to come to the Council for the sake of a theological discussion. There is nothing wrong with this, since it was oriented toward their eventual conversion and it is the same case with the 1949 decree under Pope Pius XII.

Link1 and Link2.

I think the correct view is that, just as in government functions or other public places, Catholics are allowed to recite a formal prayer and those invited may join in, praying with Catholics in a Catholic prayer, but not praying along in a false heterodox prayer in a heretical service, this can be allowed for the same reason.

Consider these statements from the Council of Florence:

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Thus, the former council of Basel debated the choice of a place for the future ecuмenical council. Those on whom the power of choosing the place devolved, passed a decree which was accepted by the ambassadors of our most dear son in Christ John, emperor of the Greeks, and of our venerable brother Joseph, patriarch of Constantinople. Some persons chose Avignon or another place, but the said ambassadors protested that most assuredly they did not want to go there, declaring as certain that the said emperor and patriarch would by no means go to the said sacred council unless we attended in person. Those who asked for Avignon, afraid that the Greeks certainly would not come to them, dared to concoct a certain decree or notorious pamphlet, which they call a monition, against us, even though it is null and indeed leads to serious scandal and a split in the church, disrupting this holy work of union with the Greeks.

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Finally, our most dear son John Palacologus, emperor of the Romans, together with our venerable brother Joseph, patriarch of Constantinople, the apocrisiaries of the other patriarchal sees and a great multitude of archbishops, ecclesiastics and nobles arrived at their last port, Venice, on 8 February last. There, the said emperor expressly declared, as he had often done before, that for good reasons he could not go to Basel to celebrate the ecuмenical or universal council, and he intimated this by a letter to those assembled at Basel. He exhorted and required all of them to go to Ferrara, which had been chosen for the council, to carry through the pious task of this holy union.

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Since in fact the plague continues from day to day and it is feared that it will gain strength when spring and summer come, all judge and advise that a move must be made without delay to some non-infected place. For this and several other good reasons, with the agreement of our dear son John Palaeologus, emperor of the Romans, and of our venerable brother Joseph, patriarch of Constantinople, and with the approval of the council:

http://www.ewtn.com/library/councils/florence.htm

It would have been strange to refuse a recitation of the Lord's Prayer with individuals whom you had previously referred to as being a "dear son in Christ" and a "venerable brother."

Yes, at least materially.

Quote from: SJB

Quote from: Pelele

This decree is regarded as a condemnation of false ecuмenism and as a reiteration of Mortalium Animos, and indeed it is, yet how is it that in #V it still allows for common prayer with heretics?

Quote

V—Although in all these meetings and conferences any communication whatsoever in worship must be avoided, yet the recitation in common of the Lord's Prayer or of some prayer approved by the Catholic Church, is not forbidden for opening or closing the said meetings.

How is this possible?

Reread Mortalium Animos.

Truly this is all you can ever say, "you're reading it wrong", "you're not understanding it", "it doesn't say that", "there's no contradiction", "both say the same thing" etc. etc. etc. without ever making an argument and actually explaining anything.

Re-read Mortalium Animos? Well here you go:

Quote

#9: Everyone knows that John himself, the Apostle of love, who seems to reveal in his Gospel the secrets of the Sacred Heart of Jesus, and who never ceased to impress on the memories of his followers the new commandment "Love one another," altogether forbade any intercourse with those who professed a mutilated and corrupt version of Christ's teaching: "If any man come to you and bring not this doctrine, receive him not into the house nor say to him: God speed you."

You are the one claiming the Holy Office contradicted Mortalium Animos. You've made no specific argument in support of your "theory."

The word which was used is recitation.  Once again, if Protestants were allowed to actively participate during the Council of Trent sessions, were they allowed to be present when the Lord's Prayer was recited, which no doubt occurred just prior to the opening of any day's session?