Hello

You wrote an excellent Wiki of very high quality.
But there is an error:

IMPORTANT NOTE 1: There is a bug/limitation in WinUSB that will force
the wIndex of any Interface Request to the interface number. This means that,
if you are using WinUSB to verify the content of your Extended Properties Feature Descriptor,
you won't be able to retrieve it (unless it is only defined for interface #5).

This is not a bug.
It means only that you did not understand how Microsoft has designed WinUSB.
Please read my posting how to get WinUSB sending a SETUP request to any interface:

libusb/libusb#1675

I can send a SETUP request in my code to any interface of a Multi-Interface USB device without problems.