Q: Defective iPhone 6, can't reset and non-responsive

I bought a refurbished iPhone 6 from an online store. I connected it to iTunes and went ahead and restored from my old iPhone - so all my settings and content now exist on this new phone.

Unfortunately, I didn't realize before I did that that this phone does not work! The screen is almost completely black, and it's non-responsive - I can (barely) see the "slide to setup" prompt, but it will not accept any input. Holding the power button triggers the "slide to power off" screen, but since it's unresponsive, it won't accept my swipe. Holding the home and lock keys down does trigger a reboot, but it's just the same situation again.

Obviously I need to return it - no big deal. But I can't return it without resetting it and removing my iCloud account from it...and iTunes will not let me reset the phone without turning off "find my iphone"....but that has to be turned off on the phone itself, which I cannot access!

Is there any way to reset this phone? I tried logging into icloud.com to see if I could "disconnect" the phone from my account from there, but it doesn't show up in the list of devices associated with my account