Remember the golden rule of statistics: A personal sample size of one is a sufficient basis upon which to draw universal conclusions.
Upload the computer to Dropbox and provide a link to it so others may download it to examine and give advice for repairs.
EL OH EL what is even the point of a plastic backplate?
Aesthetics I would imagine. I mean, sure it doesn't do anything for thermals (which to be fair is supposed to be the point of a backplate) but most of the metal backplates really don't do anything for thermals either. I dunno, I'd always prefer the back of the PCB to be covered by anything that doesn't look like the back of a PCB, just for looks personally. Kinda like the plastic "Thermal Shielding" on Asus mobos. They don't do anything for performance (and sometimes make thermals worse), but I still love the clean look and design of them anyway. I still have the very first board to use the thermal shielding (P67 Sabertooth) and I can say without shame that that was the main reason I chose that board back in the day.
I know, I know, flame away guys, but I've always been completely forthcoming about my penchant for aesthetics (even empty aesthetics) and its the same reason I love RGB and tempered glass side panels and all the other "silly" decorations PC's have these days.