That's a horrible situation to be in. The 2nd phone you received should have gone through warranty replacement through apple, as the 30 days buyer's remorse period had passed I believe. The 30 days for online and phone orders begins from the day the phone was shipped. Regardless of date, you were told and sent out a shipping label. That should be proof enough for you to get the iphone. 

If I were in your shoes, I would escalate further to speak with a manager who isn't afraid to ask for an iphone exception, or an easier solution if you have received the refund for the phone itself, is to try to find a manager who is willing to do the upgrade reversal. It's much easier for a manager to do an upgrade reversal than do the iphone exception order. The proof that you had returned your phone would have been more than enough for my former manager to do the reversal for you.