You really make a good points here. Thanks for the effort on contributing to this thread! I just want to discuss some points that you made.
Color Scheme
I highly agree on this one. I love dark theme too, but the way Microsoft implement it and interpret it is quite misguided. I don't think every app and every window element should only have a pure black background. I agree about the principle why production apps don't use pure black UI, but rather a dark gray instead in any intensity. There is one app that I used called Polarr and it gives you several theme options, I chose the Dark Theme that has dark gray background instead of Dark Theme+ that have pure black, the reason is what you've already stated, especially when you are actually working on something and not just consuming stuff.
I think the reason why Microsoft just got stuck to pure black background is because of Windows Phones, which was by default got black background. It was good especially for AMOLED, and fitting for a device that only shows one app at a time and the design language makes the UI like floating or laminated on the screen. Metro Design Language principle is being digitally authentic, not pretending like it something else entirely. That's why the feel of it is as if the UI really belongs to the screen, to the glass itself since it blends so much. Though we know we are moving away from it, and for desktop where multiple floating windows are ways we interact, having a pure black windows is also feels kinda bit odd at times too.
I think having 3rd-option would be nice, especially for smaller screen devices such as smartphones or even small tablets. Having pure black background on some apps are more desirable especially for one-app-at-time experience. I kind of benefit for AMOLED users too, though I don't know these days if that even matters much. Though I'm thinking for the sake of simplicity for users and even developers, we just need to tweak the app UI to have dark gray for apps that are appropriate for them. I regards to Photos and Videos app, I'm kinda split about it, but pure black has to remain on photo viewing and video playback though.
In regards on light theme, this needs to have light gray too. Seriously, the pure white is kinda too intense if used to virtually the whole window.
Blur/Drop Shadows
Putting blur to every app is indeed a concerning decision that they do on their apps. I think this has to be only used when really necessary. Though to be fair, their guidelines do specifically recommend but not strictly about using Acrylic (blur) when designing an app. The problem is that they seem to violate that themselves such as on People app, which is disgustingly horrible. If I remember correctly according to their own guidelines, you only should make the whole window of an app to have Acrylic or blur is when that app isn't going to have big window sizes, and mostly floating on top of another apps. The Calculator app is the best example of this, since this type of app that you won't normally used on maximized and mostly remained on a small window. People app on the other hand is a kind of app that shows information density. Even their guidelines advise to avoid using Acrylic on larger scale specially when it contains dense information such as text.
1px border
Where is the image? I think you forgot to add it.
Anyways, I would be the one that also want 1px window border to be removed, not just being clean but also to be more cohesive to the general UI design. Most provide visual hierarchy on UI not with 1px borders but with difference with UI panel colors. Though yes some separators has to be used when needed, which is even oddly missing on some areas such as on Settings.
Though I find it the necessity of this is less with Fluent Design when Acrylic is used. They can make it like changing to be more transparent on that material when that window is behind or inactive. Currently they are actually solid for some reason. Another thing is that window shadows already provide a visual hierarchy between windows.
Still even I want 1px border to be removed, I don't agree either on just removing just for the sake of it with current implementations. The problem is that only Acrylic material change its state when the window is either foreground or background, and Acrylic is typically a smaller portion on that window.
So I got another suggestion that I think will help fixing this issue if we want 1px border to be removed. We need other UI element to also change their tone when being inactive, but don't change the content. When that window is inactive, make the UI elements darker and little faded, only leaving the content unchanged. This should make the whole inactive window more obvious that it is inactive since most of the UI are faded compared to active window that all looks normal.
Though if we don't go on that route, then retain that 1px border, but please use the color of that app, not the system theme unless not specified. The color of the window border and the in-app UI tend to conflict which makes looks unpolished and doesn't seem to belong. When the app is inactive, then just make it black or super dark grey like what it is now.