There is no magic, macOS just checks which version of the SDK was used when compiling the app, if the SDK version is 26 or higher, it will make it go through the new UI path, if not it will get the previous UI. I don't know how the Python bridge works, but it clearly needs to be updated or recompiled with the new SDK.

I guess you have to use Bt. 1886 instead: https://en.wikipedia.org/wiki/ITU-R_BT.1886

Maybe you need to set the window style to NSWindowStyleMaskFullSizeContentView.

Yes, there are some borders. So just make the image you insert into icon composer a bit smaller, and that's it. The colors look the same here, but it could be your original image hadn't a color profile assigned, and it looked differently on a DCI-P3 display. Assign a color profile before importing it in Icon Composer.

Attach them after the CVPixelBufferCreate call. The attributes dictionary that can be passed to CVPixelBufferCreate is just to setup the pixel format, not to attach other attributes. So call CVBufferSetAttachments after the CVPixelBuffer is created and set them. P.S.: modifying CVPixelBuffer attributes is quite expensive, do it only as needed.