First of all, religions are either compatible or incompatible. Sure, one can imagine a pan-Abrahamic council based on the shared mythology and the Seven Laws of Noah. But there is no way on this green Earth to reconcile the transient Norse gods (who are amoral, mortal, race-blind and have no favorite tribe) with the eternal Abrahamic god (who is a Victorian prude, immortal, and, at least in some versions, extremely biased towards a favorite tribe), with the variegated multiplicity of the luxuriant Hindu pantheon, etc.
There is nothing which all religions have in common. Moreover, some religions, such as the faiths of the Abrahamic family, explicitly state that their One True Living God is the only god, and all religions which dare worship any other gods, or, God forbid, goddesses, are mortally sinful.
Lofty ideas about inter-faith councils crash as soon as they are confronted with the sad reality that religions are religions, and at least some of their followers actually believe. Let me give an illustrative example.
The Council of Florence
The Council of Florence (which began as the Council of Basel) was summoned by Pope Martin V in 1431; among other noble goals, it aimed at restoring the unity between the Western Catholic Church and the Eastern Orthodox churces, unity which had been shattered by the East-West Schism in 1054.
Apparently, this should not have been that difficult.
At that time, the only doctrinal difference between the Catholic and Orthodox churches was the Catholic doctrine of the Purgatory; the Orthodox were known to practice intercessory prayers for the dead (we still do), so it was to be assumed that they would accept to align their doctrine with their practice (we still don't).
The Orthodox bishops had a strong incentive (that is, the Turkish stranglehold on Constantinople) to agree to the reunion; and they actually agreed to come to the Council.
Satisfactory fudges had been found around the vexing practical differences between the two almost identical variants of the Apostolic Christian faith.
The Council deliberated for eight years, but, in the end, in 1439, (almost) all the bishops present had agreed to the wording of the Papal Bull re-uniting the Western and Eastern churches. And then...
Agreement was reached on the Western doctrine of Purgatory and a return to the pre-schism prerogatives of the Papacy. On 6 July 1439 an agreement (Laetentur Caeli) was signed by all the Eastern bishops but one, Saint Mark of Ephesus, who, contrary to the views of all others, held that Rome continued in both heresy and schism.
To complicate matters, Patriarch Joseph II of Constantinople had died the previous month. The Greek Patriarchs were unable to assert that ratification by the Eastern Church could be achieved without a clear agreement of the whole Church.
Upon their return, the Eastern bishops found their attempts toward agreement with the West broadly rejected by the monks, the populace, and by civil authorities (with the notable exception of the Emperors of the East who remained committed to union until the fall of the Byzantine Empire to the Turkish Ottoman Empire two decades later). (Wikipedia)
This example shows that even in the best possible conditions, inter-faith councils are doomed to fail.
Let a thousand flowers bloom
The first thing to recognize is that religious wars are rare. Think of the entire classical Antiquity, spanning a thousand years from the Battle of Marathon to the fall of the Western Roman Empire: how many religious wars were fought during this time? None¹. How was this possible? Easily: nobody even thought that religion was a suitable cause of war. Everybody knew that different tribes had different religions, because they were different tribes. It made no sense for, say, Rome, to try to impose its religion on, say, the Egyptians; Egyptians were Egyptian, they had their barbarian Egyptian gods, which were suitable for Egyptians.
(Note 1: With the possible partial exception of the Second Jewish War, also known as the Kitos War; this rebellion or civil war may be seen as having religious underpinnings, at least in part. However, it is better seen as an ordinary ethnic conflict.)
The second thing to recognize is that, although rare, religious wars do happen from time to time, and when they happen no talk shop is going to put out the flames. The initial phases of the Arab conquest, the First Crusade, the initial phases of the 30 Years War had strong religious motivations. Can one imagine that with enough skill and patience these wars could have been avoided by defusing the tensions with soothing words?