These solutions were already readily available in other documentation and did not work. As I stated before "it appears that I need to wipe out the Win10 device's previous association with the BT keyboard so that I can start the pairing process again, but I cannot find any documentation on how to do that."
The device is NOT currently listed as a under "Bluetooth & other devices", so it can't be removed. However, despite not being listed under Bluetooth & other devices, the devices is being treated by my phone as if it is already paired and is simply asking for the existing PIN.
There is no default PIN for this device. As I noted before, when I connected to an Android device, it generated a PIN. I also attempted to connect the device to my Win10 laptop and it also generated PIN number (see attached image).
The problem is that I have previously attempted to pair my Win10 mobile device and the pairing process timed out before I was able to enter the PIN. So it now appears that it is expecting me to type in a PIN that my Win10 phone previously briefly gave me, but for which I do not remember and had no means to record.