Well, Lewis came to Christianity via a form of Idealism, and as such, I don't think the difference between an 'imagined' or representation of 'reality', and Reality is such a stark divide. Lewis frequently intimates that myth or spiritual desires are in some way 'more real' than the more quotidian experience. As in his afterlife fables, things become 'more real' the deeper you go. To me, I know that my perceptions are all mediated via sense organs and nerve function if physical reality is in fact true intersubjectively, so functionally what appears real is merely my own abstraction drawn from this data. So even if true, it is perceived in the form of a mental abstraction, so the difference here between an divine allegory and its ontologic actuality doesn't seem that stark to me - but then again, I am a Protestant...
This has to do with Plato and Aristotle though, as Lewis notes, the more transcendant and the immanent, which you also connected to Pantheism and Deism. This is perhaps mirrored theologically, I agree. The ideas of a 'Spiritual Reality' beyond our own, of our actions mirroring such a deeper activity there, sound suspiciously like Plato's Cave. Our symbolic actions the shadow of deeper meaning there. As such, to think one can 'know of God' by watching the shadows dance perhaps seems understandably weird. If my spiritual perceptions are more akin to what is divine, with the added advantage of not being so encumbered by mere fallable sense-data, then trying to reconstruct the 'objective thing' casting said shadows should stress it more. For the world as we perceive it is of necessity distorted - in fact, with nerve modulation or psychologic bias and such, even physiologically we know it to be so. But then, we are back to fighting old battles, of inductive Forms we assume vs those extrapolated from the raw data we receive. What takes precedence and how do we establish epistemologic truth?
Lewis is a Protestant, and is clearly viewing this from a Protestant lens, if his schema is correct. To a Protestant, Catholicism is thus very allegorical toward spiritual truth; but from the Catholic viewpoint, this is actual reality as you noted - as the Eucharist makes very plain as an example. Perhaps this is more an example of the Ancient Unities of Barfield, where conceptually the spiritual 'essence' and the physical act has been separated by Protestantism into symbolism, while Catholicism left them undifferentiated - more akin to ancient ritual, and as Lewis noted, true allegory exists before this bifurcation really occurs. As in all things, the paradigm we bring to the table impacts the resulting conclusion we draw.
But this is a difficult problem. On reading this piece of Lewis', I noticed a lot lining up nicely to it, but there is a lot to digest. This was more an aside in a literary study on Allegory, than a theologic position, so there are many wrinkles to iron out.