I think
David Edelstein's write-up sounds like it fits pretty well with your notion, though you may need to adjust some of the back story and Choir. Perhaps instead of being an Outcast Grigori who's still tacitly tolerated, he's the quiet, secretive Cherub you describe, still in good standing.
(One reason I personally like the idea that he's a Grigori, though: No Grigori Archangels are actually described by name in the canon -- just noted as "the Archangel of Song" -- and the Archangel of Death is never named either. Perhaps, just as the Seraphim Council believes that speaking the Outcast Choir's name disturbs the Symphony, it's considered bad form to refer to the Outcast Archangels by anything but their Words...)