The cherubs of that architectual sky were pigeons, so far overhead in their flutter from roost to roost that they were only faintly discernible.
2)What type of modifying phrase is in italics? I know that it isn't a summative modifier as it isn't a noun being renamed... It seems to be a phrase that doesn't flow on from the preceding clause.
1)And can one basically add any phrase one likes onto the end of a clause PLUS a comma, as long as it is clear what it modifies? I see this so often in books etc
I think it's an adjectival phrase modifying the metaphorically introduced noun pigeons. Birds, after all, can flutter from roost to roost. I think you can use it wherever it is clear, though I'd use it directly after a noun or a verb.