There’s a problem with your proposed dialogue, and it may be insurmountable.
You think, for the various reasons you’ve sketched, that I’m wrong about the “transcendent unity of religions”, and you wish to argue me out of my silliness. But I have no interest whatsoever in persuading you I’m right. I don’t wish to change your mind, whereas you have a motive, and perhaps (from your point of view) an obligation, to change mine, if for no other reason than to protect other Christians from a distorted presentation of the Gospel.
If you still wish to talk, one possible solution might be to set the card-carrying perennialists to one side and ask instead: Was Meister Eckhart a Christian? I pick Eckhart as an “extreme case”, though one might also choose Jakob Boehme (Protestant), Angelus Silesius (Catholic), or perhaps Dionysios the Areopagite (Orthodox).
My hunch, you see, is that the real issue between us is not so much the salvific validity of other religious traditions but rather the orthodoxy (small “o”) of Christian metaphysics. In a sense it’s Alexandria and Antioch all over again. Needless to say, you’re not alone in thinking that such figures are not truly, or fully, or authentically Christian, and of course if they’re not, then I’m not. One thinks of Luther’s comment that Dionysios “Platonizes more than he Christianizes”.
Is it possible for us to get over this hurdle?