That's it. I've had it. There's been numerous updates to AMD Radeon ever since I posted this thread, but even after all that, the f***n video driver won't stop switching back to pillarbox mode even when I strictly tell it to stay in fullscreen mode!
And since no one here's bothered to help me out with this, I'm just gonna boycott AMD until they finally get off their lazy a***s and fix this!
I will say this if it'll help: For a while I downgraded my laptop back to Windows 8.1, and when I did, Radeon didn't have any trouble with Display Scaling at all, even with the latest drivers. So the pillarbox problem IS linked to Windows 10 after all.
I would stay with Windows 8.1, but can't since an upcoming game I really wanna play requires Windows 10 to operate. Sorry for my foul language, but I'm really tired of this issue and just want you guys to fix it...