I Love Apostrophes
"Apostrophe is an address to the absent as if present, the inanimate as if animate; a rhetorical question is one to which no answer is expected. These figures of power. To use them is to address someone who cannot talk back-a strategy that ensures not only that you will have the last work, but that your discourse will manifest a high degress of "'literariness'" (Garson 453).
It seems to me that Keats used apostrophes to allow for an unanswerable, unsolvable rhetorical question about the urn and the overall poem. Since the narrator speaks to her, the figure as it is noted that the urn is personified as a female, by apostrophizing her, he calls her into being; he tells her history even as he laments her citizens' inability to tell it (Garson 458).
This lack of identity to the urn/female as well as this dream of cultural possession shows a dream or desire of supplementing our lack with a borrowed integritas (Garson 458). The urn speaks to the spiritual ear in search of an answer that the urn/female as well as the readers are looking for to explain not only the urn but the overall theme of the poem rather than the material ear.

I think you are hitting on something when you dicsuss the urn's relationship to spirituality.
The urn is showing us as story rather than telling. It is sharing rather than giving. How is that not spiritual.