Thought I'd chime in here about the QC sticker. I work for a retailer who is an AT&T authorized reseller, and I've seen a few phones come in with similar QC stickers before. The factory seals have been cut and the QC seal was placed over the factory seal. I believe it's just what it seems like; it looks like AT&T probably randomly checks some phones before sending them out to retail/corporate stores.
Current phone: Google Nexus 5; Current tablet: Nexus 7 (2013)
Other/Backup/Retired phones: Huawei Vitria, Alcatel OneTouch Evolve, Google Nexus One, T-Mobile G1, HTC One X Evita (broken SIM slot ), Nokia Lumia 520; Retired tablet: Amazon Kindle Fire (1st gen)