First of all awesome work 
I want to tag along the PBR discussion.
PBR makes material texturing easy compared to Specular and Glosiness where the value can be arbitrarily out of range.
The only thing people don’t realize in using PBR are the things that supports it.
HDR Reflection value and projection are key to PBR.
In an ideal world of PBR you got perfect 1:1 HDR reflection projection and life will be great.
Problem is it’s very costly to have a perfect realtime HDR reflection.
In UE4 they augment Basic cubemap projection with Screenspace reflection, but even that is not enough cause it didn’t cover your back camera view changes properly.
This is where Specular from an Area lights comes in handy and become another important property of PBR.
But then again currently UE4 only supports spherical area light (I believe you can strecth it horizontal/vertical) but never a rectangular panel.
This is ok if your light source is always a sphere, but when your light source is a panel it becomes a problem (like the TV).
The issue can be seen on your image 6 that you posted.
The screen cast screen space reflection, but it’s value is not intense enough and it didn’t cast shadow unless you bake lightmass, to solve this you place another light that creates a Circle Specular on the floor which failed the PBR because there’s more energy emitted from the TV than its supposed to do (Failed in terms of Physical property coherency), to be CLEAR, I didn’t mean to say it’s bad as a lot of times it’s artistic choices
In any case to summarize, PBR depends heavily on Reflection that has proper light ratio and proper light area setup.
Another issue with PBR is the manipulation of specular color.
The first thing comes to my mind when I tried PBR is iridescent material. I tried to create an oil slick material with it.
Good thing Unreal comes with a plethora of material nodes.
I use Fresnel on metal parameters and gradient ramp to get it to look like an oil slick.
The point is, it’s not as simple as it used to be using specular color system, but certainly doable, just have to think differently