The baby SOI also will help to kick nic when he was down. for centuries, nic acted to destroy knowledge about his order. I am betting the kid inhereted alot from her mother, and her father has soul fire, the warden of demonreach, came into this world a star born and is the winter knight, who helps the knights of the cross. He could be the very worst person to have this information, it was bad enough that he knew the noose was his weakness, now he potentially knows a vast array of similar details, and he has locations where nic can not see into.
Harry could get the info on to the internet so the whole world could know the truth about the order, once on the internet, it would be near impossible to remove. It seemed to me that soulfire magic are very effective on denarians, want to bet there are alsorts of spell or rituals know to the angels, that need soul fire. Names are important, so are sigils to angels, harry possibly has all of them. Look at what harry did with knowledge about nics shadow powers and places where it did not work.
What would nic do if harry got deidre's coin and put it in the well, possible any denarian coin? It seems like it is the one prison capable of holding denarians. WHile i hope the whole order gets sealed away, it seems likely that some will, just enough tomake the denarians a lesser threat. The named denairan are the biggest threat as those with out are seemingly just minions.