DJMirror allowing screen data from S8 / S5 / D2 to be displayed on external monitor

No, unless you put in additional code for a global cross-device switch both for X1MK2 (via overmapping, or using Browser-Only Layout) and the D2/S8 code (qml, for the device mapping loaded to create the screens) that will switch to ScreenBrowser internally.

Regarding a) and c)

The mod creates as many deck windows as you want (with mounting number of mapping files), each window must be placed and sized manually (with content resizing, depending on the graphic element definitions), which can be a bit fiddley.

Several different waveforms can be selected (including stripe), much like i did for the S4. The creators have collected a whole slew of colour definitions (i found several of my own creations in there). All these can be accessed via a custom setup, which will display on the new screen windows, but so far can only be accessed when using D2/S8/S5, although i read they plan to add this (and other) features to the rest of NI (qml based) controllers, which have remained untouched for now.

Their current focus is touch screen controls for use with macOS.

True, but you can use mapping commands to toggle Traktor variables like for example Browser-Only Layout. Those in turn can be read by qml code and can be used in the D2/S8/S5 deck code to trigger the screen browser.

I'll have a look at where to place the code. It'll be a simple switch, and so it will trigger screen browser globally (on all visible s8-style screens: s8/s5/D2/DJMirror-windows.)

Did not yet work the way i hoped, not sure why. I thought it must be easy, but it looks like the data generated by the code is not being transfered to the relevant screen sections. More tests needed with my D2's when i unpack them. Testing everything s4 currently.

Somehow i believe the creators will get around to the other NI devices (X1MK3, S4 etc.) not too far aways.

NOTE: Nobody has been talking about this and it's written nowhere i looked. Each window has a mouse function. Click somewhere in the Deck Header. You will get a simple menu, including switching top/bottom screens (View), screen browser (you can select and load tracks with the mouse) and Config Setup Menu.