Yes, I certainly do believe that there is, or at least can be, an advaitic form of Christianity; as a matter of fact I’m at work on a book on the subject. I realize this may come as something of a surprise. But as I see it, that’s because you mistakenly think that a non-dual formulation of this, or any other, tradition must be to the exclusion of the hierarchical—and seemingly dualistic—point of view that you associate with neo-Platonic metaphysics. On the contrary, it seems to me that these are two equally plausible perspectives on the same apophatic Reality. Neither is “right”, and neither “wrong”.
It all depends on what a given person means by (your phrase) “the ordinary world”. You point out that the Bodhisattva sees “no difference between Samsara and Nirvana“. True enough, but is the Samsara he sees the same as the Samsara I see? That’s for me a key question. And the answer surely is no. Doesn’t the fact that he sees Samsara as non-different from Nirvana mean that he sees it more truly or clearly than I do? Otherwise what would be the point of this central Mahayanic teaching, if not to help me see in a new, more authentic way?
As I understand it, the hierarchical point of view of neo-Platonism and perennialist metaphysics, culminating “at the top” in an ens realissimum, has the soteriological function of helping draw me out of the old and “up” and “into” a new vision, a new level of experience. But once I have entered upon this higher level and have gained a certain distance on the world as ordinarily viewed through the screen of my habits and attachments, I will see that this ens realissimum, being necessarily infinite, cannot be other than “the ordinary world”, but an ordinary that is now in fact truly extraordinary.