If you use the plastic case that comes with the phone don't use it the tolerance aren't great. Its also maybe the culprit of bending the phone that the backglass become lose. 

There is no root. Because the bootloader can't be unlocked.

Maybe that what cause the backplate to become lose in overtime. Or that can be the problem with the Air triggers. Who knows?

Mine has a small gab like less then a 0.5mm but i can't sqeeze it. I found something more interesting. You should take a look at both corners on the top. It looks like the backplate is slightly misaligned more to the USB side of the phone. Maybe that...