Gathered all four Horde flames. Started the quests from the quest items. Was able to turn in Orgrimmar and Silvermoon City, but…
“Stealing the Undercity’s Flame” and “Stealing Thunder Bluff’s Flame” cannot be completed as a result of a “Duplicate item found” error. Tried deleting the quest and restarting from the quest item that starts the quest; still cannot complete. So much for finishing the meta. 
And the issue is: if you choose Handful of Burning Blossom as a quest reward, they have a inventory quantity cap of 20. THIS is what prevents the other turn-ins - delete from inventory then try again. Error message needs to be a little more descriptive beyond “Duplicate item found”.