I can see from your letter that there are several rather serious errors in your thinking.
First of all, you need to come to terms with the fact that all religions are made in such a way that their adherents think them the best, if not the only, means of salvation; this is natural, and indeed necessary, and a matter of divine dispensation. As I have said several times in this forum, a church qua church is not in the business of promoting ecumenical dialogue; it exists in order to transmit saving teachings and sacraments. Of course one finds “narrowness” and “dogmatism”, but a genuine metaphysician is not surprised by this. Indeed if you don’t find at least some resistance to the claim (in your words) that “the great religions are equal”, you can be quite sure you are not really dealing with a great religion!
As for your question regarding the Lord’s Prayer and the Trisagion, I would say no: these petitions are much too discursive to serve as a basis for quintessential orison or Prayer of the Heart, nor is there any traditional precedent for using them in this fashion. You express concern that, unlike the Trisagion, the Jesus Prayer does not “include the Trinity”. But the Name is the Named, and according to the Scriptures, the Named in this case—that is, Jesus Himself—is the embodiment of “all the fullness of God” (Colossians 1:19). There is therefore nothing not included in this Name … so long as one looks along and not at it.
You’re also bothered by the fact that, unlike the Lord’s Prayer, the Jesus Prayer is not traditionally used by Catholics and Protestants, but this is of no importance whatsoever. You’re making another serious mistake if you think that the “transcendent unity of religions” means there is likewise some “equivalence of Christian churches” (again your words). Schuon is crystal clear on this score: “Only the Eastern Church maintains the Christic message in perfect equilibrium” (see my anthology of his writings on Christianity: The Fullness of God [World Wisdom, 2004], p. 91). He certainly had the highest respect for traditional Roman Catholicism—not the Novus Ordo church!—but he did not put it on the same level as Orthodoxy, which he regarded as “the most direct, the most ample, and the most realistic” expression of the Gospel (see my new edition of Christianity/Islam: Perspectives on Esoteric Ecumenism [World Wisdom, 2008], p. 29). “The Orthodox are right about everything,” he once told me, “except”—he added with a smile in his eyes—”for their interminable liturgies!”
You say that “perennialists are supposed to uphold the universalism of the spiritual life”, but once again this is a grave misconception. I would ask you: who of the perennialist authors you have read supposes this, and upon what is such a supposition based? The fact that there is a “transcendent unity of religions” in no way implies that every expression of every such religion is “created equal”. Being a perennialist does not mean jettisoning one’s powers of discrimination.
“The people who like us,” one of Schuon’s Sufi disciples once observed, “we don’t like, and the people we like don’t like us!” A paradox, to be sure, but one that cuts straight to the heart of our perspective. I would encourage you to ponder it.